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
Merge branch '2510-add-system-db-handlers'

Fixes #6