benchmark: make it possible to configure read and write buffer sizes

[easwars]

This issues addresses 2-4 from #5751.

Our benchmarks set server and client write buffer sizes to 128KB.

• Do not set the read and write buffer sizes by default. Instead let the benchmark client and server use whatever defaults grpc uses.
• Provide a command line flag to set these buffer sizes.

[atollena]

#5762 for the first item.

[atollena]

#5774 -- @easwars I think you can assign this to me, I hope to close this issue with this PR.

[atollena]

@dfawley any chance you could have a look at the linked PRs #5762 and #5774 when you have time? The first should be a quick one, the second is also probably an easy review.

[atollena]

I think this can be closed since #5774 is merged.

[easwars]

Fixed via #5774.

[easwars]

@atollena Thank you very much for your contribution !!