nexus shattering 3 of 3: Extract nexus-db-model crate from nexus::db::model

[jgallagher]

This is "nexus shattering" PR 3 of 3 (for now), which splits the database model into its own crate (nexus-db-model). In addition to the files that were previously under nexus/db/model/, I moved db/schema.rs, db/ipv6.rs, and db/saga_types.rs into nexus-db-model. I moved them primarily because other modules under db/model.rs depend on them, but I think all three of those arguably fit with the model logically anyway; I'd appreciate it if someone could confirm/deny that.

TL;DR: This PR (together with the 2 previous ones) speeds up several cases of both incremental and clean builds by 17-25%, with middling improvements (3-7%) to the remaining cases. At least as of this PR (and arguably before), incremental build times are dominated by linking, and using a (much) faster linker can decrease them by another 50+%. More numbers and analysis below the fold.

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Operation	main (d023a6d9)	this branch (454959e0)	this branch using mold
clean build, lib+bin	2m30s,2m29s,2m33s	2m19s,2m24s,2m21s	1m59s,1m59s,1m59s
incremental build, lib+bin, datastore/mod.rs modified	36s,36s,37s	29s,31s,31s	12s,12s,12s
incremental build, lib+bin, model/image.rs modified	36s,36s,37s	35s,35s,35s	17s,17s,16s
clean build, tests	4m39s,4m39s,4m31s	3m45s,3m44s,3m37s	3m17s,3m19s,3m17s
incremental build, tests, datastore/mod.rs modified	1m5s,1m4s,1m5s	48s,48s,49s	20s,21s,21s
incremental build, tests, model/image.rs modified	1m5s,1m5s,1m5s	52s,52s,52s	25s,25s,26s

As with previous timings, in this table "lib+bin" is cargo build -p omicron-nexus and "tests" is cargo build -p omicron-nexus --tests (i.e., the build of the tests without running them). "Modified" means the a space was inserted at the top of the file (which can cause more work than just touch, thanks to issues like rust-lang/rust#47389). Modifying model/image.rs on this branch triggers a rebuild of both nexus-db-model and nexus, as opposed to modifying datastore/mod.rs (which only triggers a rebuild of nexus, not nexus-db-model).

New as of my timings on this branch: I did all the above timings using a 32GiB ramdisk as my target directory and repeated them 3x to check for variance. Before switching to a ramdisk, I was seeing a lot of wild variance and even outright failure (#461) which appears to have been due to poor filesystem performance. A very full filesystem or an ext4 filesystem on an SSD that has had a lot of churn since the last time fstrim has been run can have an outsized impact on build times (I've observed as much as 2x the numbers in the table above for successful builds, plus the builds that failed outright).

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I was very surprised to see the full build including tests speed up by nearly a full minute. Repeating that build on both main and this branch (using the stock Linux linker) with --timings, here are screenshots of the end of the build process with the min unit time set to 1.1s and the scale set to 5.

main:

this branch:

It looks like there are two factors:

1. The combined compilation time of nexus-db-model and omicron-nexus on this branch is roughly the same as the time to build omicron-nexus on main, but compiling nexus-db-model is able to start sooner and concurrently with other dependencies of omicron-nexus. (It's hard to tell from the screenshots, but I also think omicron-nexus is able to start slightly before nexus-db-model completes; perhaps rustc doesn't have to wait for the full codegen of a dependency before it starts building the dependent?)
2. The time to build omicron-nexus lib (test) (the final and longest crate built) decreased by over 30 seconds.

The second point made me realize this is perhaps not a fair comparison, although it might still be the most useful one. On this branch running cargo build -p omicron-nexus --tests no longer builds any tests that moved into the shattered crates. If one is working in nexus proper and not touching the shattered crates, that's probably a good thing. But for a more fair comparison, perhaps cargo build -p omicron-nexus -p nexus-db-model -p nexus-types -p nexus-defaults --tests would be better? This is only a couple seconds slower (3m57s), again due to the shattered nexus-* test crates (which take a combined ~36s, ignoring parallelization) being able to be built concurrently with omicron-nexus itself:

[davepacheco]

What a huge improvement! I've got one minor comment below and also asking if there's a TODO here you meant to address before landing this. Anyway, this is looking great and seems like it'll be a big quality of life improvement.