(#956) - Inter HTTP Replication

Initiate a CouchDB replication via `_replicate` checkpoint via `PouchDB.replicate`
api when both source and target adapters are http.

All tests pass.

One caveat: inter http replication supports `onChange` callbacks via changes feed. That way the response is different from the one provided by Pouchdb.replicator.
Also, `onChange` is only used when replicating in `continuous` mode. The correspondent test is skipped for inter http.