Replication performance improvements

#### (#986) - Faster Cancelling of Continuous Replications 
_Donno why its in the diff as it has been merged (https://github.com/daleharvey/pouchdb/commit/c967a6b5e8a811b77916e05b4e34d788ebcb7021) but its not in master nevertheless._

#### (#932) - Use bulk API for _revs_diff
Queue revs diff requests and and send them in bulk.

#### (#993) - Use allDocs to batch generation 1 docs
Batch fetching docs of generation 1 via `allDocs` api in replication.
* handle deleted docs correctly (they could have been deleted via POST with
`_deleted: true` without removing data)
* Handle docs, which revision has changed after change has been emitted