_bulk_get implementation

Nonstandard REST API that speeds up client pull replications.
(It also needs to be implemented in Couchbase Lite.)
Fixes #45.
Update Walrus: incremental view indexing

This makes Walrus a lot more useable again with non-tiny databases.
The only real slowdown is the first query of a view after launch, when
the entire index has to be built. After that it's fast.