ISGR: filtered push replication replicates channel removal revisions

Completed: https://github.com/couchbase/sync_gateway/pull/5076
Sync Gateway Branch: CBG-1454

There's been a report that filtered push replications are sending revisions of document that leave the channel.

Needs further investigation, but may be due to active peer running as admin, as follows:

• active peer pushes 'removed:true' revision via changes
• passive peer requests rev
• active peer checks whether they have access to rev. Since they are running as admin, will always have access
• full revision gets sent, even though it doesn't match the channel filter

A possible fix would be to not send removal changes messages in the first place for ISGR, since we don't support purge-on-removal for push replications in any case. Need to review whether purge-on-removal makes sense for push.