[multiplex] merge flush and use sharded hashmap #321

[miamia0]

Motivation

optimize the multiplex transport

1. use shared hashmap instead of Mutex<Hashmap> to reduce race
2. merge the flush in the write loop

Solution

[CLAassistant]

All committers have signed the CLA.

[choleraehyq]

shared -> sharded?

[miamia0]

@PureWhiteWu PTAL