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.

backport change from couchdb-couch-replicator
https://github.com/apache/couchdb-couch-replicator/commit/75e5ba177431483522f983ffab305213f0ccbfe6