[CCR] Limit the write buffer by memory rather than op count

[bleskes]

Currently we stop sending read requests if the number of operation waiting to sent as a write bulk is higher than a given number. That was the simplest to implement but it makes it harder to reason about a good value - the ultimate goal of setting a limit is to protect against memory consumption. I think we should move to a byte sized based limit.

/cc @jasontedor @martijnvg

[elasticmachine]

Pinging @elastic/es-distributed

[jasontedor]

This was part of the initial plan, we should implement this now indeed. I discussed this with @martijnvg in another channel and he will proceed with the implementation.

One thing I'm unsure of is this should be a move to a byte-sized based limit as opposed to having the possibility of either (and enforcing both). I lean towards the latter.

[bleskes]

I'm fine with having both but I think a bytes (i.e. memory) based one is enough. I think it's the main motivation to limit it. I struggle to find a good default for the count based one, but it's easy to have sane default (1GB or 512MB) for the bytes.