Using PrepareMsg API in ServerStream

[eafzali]

Use case(s) - what problem will this feature solve?

We have server that sends really large payloads, our benchmarks shows that most of the cpu time on server is used for serialization. we tried caching the PreparedMsgs and sending them and we saw a huge improvement.

Proposed Solution

#3480

Alternatives Considered

We are now using a custom Codec and send []byte in stream.SendMsg() as a workaround.

Additional Context

#2432

[easwars]

Assigning to @dfawley who is the assigned reviewer of #3480.

[easwars]

Assigning the fixit label to make a decision on whether we want to proceed with this change or not.