[DISCUSSION] _bulk_get support for the replicator

[Nick Vatamaniuc <va...@apache.org>]

Hi everyone,

I implemented _bulk_get support for the replicator in
https://github.com/apache/couchdb/pull/4144. I tried to make it as
backwards compatible as possible to replicate with older endpoints and
PouchDB. Some preliminary benchmarks showed a decent speedup of 30% or
so.

There is a silly chart in the PR comments showing the effect of the
exponential moving average to make it less mysterious where some of
those constants came from. I had opted to make some of those constants
hard coded, to avoid adding even more replication options, but can be
convinced otherwise. Feel free to suggest or comment in the PR.

Also, to add some visibility regarding _bulk_get effectiveness I added
two replication stats fields. Not sure I picked the best names, let me
know if you have any better suggestions or perhaps not have them at
all?

Thanks,
-Nick