Feat: implement partial map-reduce

[darthunix]

Extend ref-map-reduce API with a new partial execution method:

vshard.router.partial_callrw(bucket_ids, func, args, opts, callback)

It allows users to call a function exactly once on all masters that contain buckets from the list.

Also current PR improves the connection establishment for all ref-map-reduce methods.

[Gerold103]

Finished in #491.