kvserver: refactor validateReplicationChanges()

The function had become very hard to read over time as we started to
support explicit voter<->non-voter promotions/demotions of replicas and
rebalances of replicas within a node. This commit re-writes the func
as a series of disjoint checks, each focussing on a specific class of
invalid changes. This should make the function easier to
understand/debug in the future.

Additionally, this commit also converts `TestValidateReplicationChanges`
to a table-driven test.

Release note: None