Clearing sliceCache should also throw away timeStep

[iain-buclaw-sociomantic]

A common maintenance task would be to start writing out a new stat, only to find that we want more coarse or finer precision for it later down the line.

Carbon can reload it's storage schemas no problem, and we have plugins that can rewrite metadata files to sync with Carbon config, merging any "orphaned" slices in the process. However CeresNode keeps the timeStep in memory and will continue to create slices at a retention that no longer matches what Carbon or the CeresMetadata on disk says about the node.