This commit adds a safeguard inside
`Replica.maybeLeaveAtomicChangeReplicasAndRemoveLearners()` to avoid removing
learner replicas _when we know_ that that learner replica is in the process of
receiving its initial snapshot (as indicated by an in-memory lock on log
truncations that we place while the snapshot is in-flight).

This change should considerably reduce the instances where `AdminRelocateRange`
calls are interrupted by the mergeQueue or the replicateQueue (and vice versa).

Fixes cockroachdb#57129
Relates to cockroachdb#79118

Release note: none