kvserver: partial implementation of ReplicasStorage

[tbg]

WIP

[cockroach-teamcity]

This change is 

[sumeerbhola]

Reviewable status: complete! 0 of 0 LGTMs obtained (waiting on @tbg)

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pkg/kv/kvserver/replicas_storage.go line 693 at r2 (raw file):

	// contain the term. RangeStorage must simply remove all entries > Hi;
	// additionally it's too late to SingleDelete any existing entries since
	// that write would now apply to the new entries, not the old ones.

This is used for purging the sideloaded files, which include the term in the key, similar to what happens in

cockroach/pkg/kv/kvserver/replica_raft.go

Lines 999 to 1008 in cf2b7f8

	if len(rd.Entries) > 0 {
	// We may have just overwritten parts of the log which contain
	// sideloaded SSTables from a previous term (and perhaps discarded some
	// entries that we didn't overwrite). Remove any such leftover on-disk
	// payloads (we can do that now because we've committed the deletion
	// just above).
	firstPurge := rd.Entries[0].Index // first new entry written
	purgeTerm := rd.Entries[0].Term - 1
	lastPurge := prevLastIndex // old end of the log, include in deletion
	purgedSize, err := maybePurgeSideloaded(ctx, r.raftMu.sideloaded, firstPurge, lastPurge, purgeTerm)

Maybe it would be sufficient for this to be the term of Lo?