Pending endpoints size check for CAS doesn't play nicely with writes-on-replacement

Consider this ring:

127.0.0.1 MR UP JOINING -7301836195843364181
127.0.0.2 MR UP NORMAL -7263405479023135948
127.0.0.3 MR UP NORMAL -7205759403792793599
127.0.0.4 MR DOWN NORMAL -7148113328562451251

where 127.0.0.1 was bootstrapping for cluster expansion. Note that, due to the failure of 127.0.0.4, 127.0.0.1 was stuck trying to stream from it and making no progress.

Then the down node was replaced so we had:

127.0.0.1 MR UP JOINING -7301836195843364181
127.0.0.2 MR UP NORMAL -7263405479023135948
127.0.0.3 MR UP NORMAL -7205759403792793599
127.0.0.5 MR UP JOINING -7148113328562451251

It’s confusing in the ring - the first JOINING is a genuine bootstrap, the second is a replacement. We now had CAS unavailables (but no non-CAS unvailables). I think it’s because the pending endpoints check thinks that 127.0.0.5 is gaining a range when it’s just replacing.

The workaround is to kill the stuck JOINING node, but Cassandra shouldn’t unnecessarily fail these requests.

It also appears like required participants is bumped by 1 during a host replacement so if the replacing host fails you will get unavailables and timeouts.

This is related to the check added in CASSANDRA-8346