Deflake `gossip/chaos` by adding a missing
`waitForFullReplication`. This test loops, killing a node and then
verifying that the remaining nodes in the cluster stabilize on the same
view of gossip connectivity. Periodically the test was failing because
gossip wasn't stabilizing. The root issue was that the SQL query to
retrieve the gossip connectivity from one node was hanging. And that
query was hanging due to unavailability of a range. Logs show that the
leaseholder for that range was on a down node and that the range only
seemed to contain a single replica. This could happen near the start of
the test if we started killing nodes before full replication was
achieved.

Fixes cockroachdb#38829

Release note: None

Release justification: testing change
Release note: None

Release justification: comment-only change
Release note: None

There was a bug in range quiescence due to which commands would hang in
raft for minutes before actually getting replicated. This would occur
whenever a range was quiesced but a follower replica which didn't know
the (Raft) leader would receive a request.  This request would be
evaluated and put into the Raft proposal buffer, and a ready check would
be enqueued. However, no ready would be produced (since the proposal got
dropped by raft; leader unknown) and so the replica would not unquiesce.

This commit prevents this by always waking up the group if the proposal
buffer was initially nonempty, even if an empty Ready is produced.

It goes further than that by trying to ensure that a leader is always
known while quiesced. Previously, on an incoming request to quiesce, we
did not verify that the raft group had learned the leader's identity.

One shortcoming here is that in the situation in which the proposal
would originally hang "forever", it will now hang for one heartbeat
timeout where ideally it would be proposed more reactively. Since
this is so rare I didn't try to address this. Instead, refer to
the ideas in

cockroachdb#37906 (comment)

and

cockroachdb#21849

for future changes that could mitigate this.

Without this PR, the test would fail around 10% of the time. With this
change, it passed 40 iterations in a row without a hitch, via:

    ./bin/roachtest run -u tobias --count 40 --parallelism 10 --cpu-quota 1280 gossip/chaos/nodes=9

Release justification: bug fix
Release note (bug fix): a rare case in which requests to a quiesced
range could hang in the KV replication layer was fixed. This would
manifest as a message saying "have been waiting ... for proposing" even
though no loss of quorum occurred.