revert: revert yamux buffer

[driftluo]

The throughput problem reappeared because of this commit 1ea53e9

test case: #213

Although in actual production there will be no sub-substreams above 100 levels, this is indeed a problem. After regression testing, the current code will show a significant throughput drop when it is around 800.

perf ./target/release/examples/throughput_test 800

master branch: CPU consumption of distribute_to_substream function accounts for more than 50%
patch branch: CPU consumption problem of distribute_to_substream function cannot be clearly observed, about 2.5%

This is a classic continuous memory and segmented memory overhead problem. In a test scenario under full pressure, such memory issues will be triggered frequently and cause huge overhead.

[yangby-cryptape]

The throughput problem reappeared because of this commit 1ea53e9

I want to request changes for 1ea53e9: .fold(0, |acc, item| acc + item.len()) => .map(HashMap::len).sum()