Use user timestamp to find the newest record in exist functions

[ikopylov]

This is a part of #708
It is required for correctness. We should look for the newest entries only by comparing the user's timestamps. Unfortunately, this can't be implemented properly without breaking changes in BLOB structure. So this issue for future versions.

bob/bob-backend/src/pearl/group.rs

Line 328 in 54498e8

ReadResult::Found(ts) => {

bob/bob-backend/src/pearl/group.rs

Line 335 in 54498e8

ReadResult::Deleted(ts) => {

[ikopylov]

Closed via #746