Fixed a thread-safety problem in db/notifier.go

Patch courtesy of Constantin Teodorescu.
Optimize large _changes feed by streaming response

Courtesy of Constantin Teodorescu
CouchDB compatibility fixes

* Fixed incorrect "id" property in response of PUT for a _local doc.
* Return "_id" property in GET of _local doc.
* Added "committed_update_seq" property in response of GET /db.

Courtesy of Constantin Teodorescu.
Optimize sequence number allocation

Bulk PUT now makes only one call to increment __seq, instead of one per
document.
go fmt