Don't die when a known replicator related pid dies

The couch_replicator_manager would die when any replication related pid
exited. Instead of that we don't. This means we don't cancel and restart
all known replications that happen to be running on the node due to an
error starting replications for a given database or document.

BugzId: 27666
Owner must be live

BugzID: 31712
Retry if doc is missing

The catch-all clause in remote_doc_handler is the root cause of
missing updates in replication. This occurs when we receive a
{{not_found,missing}, Rev} message instead of the expected {ok, Doc}
message, and causes us to continue blindly on, including writing a
checkpoint.

This patch changes the catch-all clause to one that handles the
missing doc case and causes a retry. Any other response will be a
function_clause which will cause a messier retry.

BugzID: 31722