[move_to_start] replication.js test race

We were grabbing _active_tasks before replication started.
[move_to_start] replication.js race condition

We were grabbing _active_tasks before replication started.
[squash] to clustered couch_changes

Delay json_req conversion until just before we pass to
couch_query_servers.
[squash] to clustered doc from multipart/mime

Fixing this hurt my soul.