Add system db handlers to fabric_doc_update

This hard codes the before_doc_write functions for replicator and user
databases like they are in couch_server. This is mostly motivated by the
fact that couch_users_db:before_doc_update/1 adds random salt values to
user documents when a password is changed. The replicator databases are
included at this level for consistency with couch_server.

COUCHDB-2510
noting that we have this in couch_replicator_manager but I can't think of a clean dedupe.