codec send Message instead of Vec<Message>

[rukai]

blocked on:
#1467
#1465

The first commit is performance neutral but for some reason the 2nd commit is heavily regressive:

From looking at a profiler we spend a lot more time in the server.rs spawn_read_write_tasks writev

I think yielding control back to the executor is preventing us from properly batching our writes, resulting in more total time spent in writev.

I have no idea why its only the writev to the client that is affected.
Oh! Maybe the writes to the DB were never batched properly?
Yep thats it!

in_w is sent a vec of a single item here.
So, we really do want to send a full message batch to the encoder.
But! If we fix the encoding for the DB connection we will have a big performance win.

[rukai]

I've reverted the 2nd commit since that was clearly a regression.
For the first commit I will keep this as a draft and evaluate this again after we resolve #1471