Allow _local doc writes to the replicator dbs

During the VDU -> BDU update we inadvertently blocked _local doc writes to
_replicator dbs. This commit rectifies that.

Add tests for _local, _design, and regular malformed docs for both the default
_replicator db as well for for the prefixed version like $db/_replicator. For
completeness, update the _scheduler/docs counts test also test both cases.
3 files changed