use buffer pool for stream buffers

[Stebalien]

1. Reduces GC pressure when creating/destroying many streams.
2. Frees memory when not in use.

This doesn't noticeably hurt throughput.

[Stebalien]

This uses https://github.com/libp2p/go-buffer-pool. The Buffer implementation is modeled after go's bytes.Buffer and passes go's test cases (with some small modifications).

[Stebalien]

After some testing, I've noticed that this does some really nice things for go-ipfs memory usage under load.

[jvsteiner]

Anecdotally, I've noticed about 50-70% memory usage reduction in my non-ipfs application.

[jvsteiner]

what about this one? it's cool...

[Stebalien]

@jvsteiner I need to think about it a bit. Go's buffer implementation avoids shrinking buffers unless something tries to read from an empty buffer. It uses that as an "idle" signal. Unfortunately, that doesn't kick in in yamux because yamux remembers how much data is in the buffer and avoids reading.

However, my implementation shrinks the buffer if the amount of data in it shrinks by 8x capacity. I'd like to test:

1. Having yamux intentionally read from the empty buffer (instead of checking Len() == 0) to see how that performs compared to this library (both in terms of GC pressure and memory usage).
2. Using the same "shrink on empty read" strategy in my custom Buffer.

[Stebalien]

I've tried the same "shrink on empty read" strategy and it really doesn't make a difference. This works and has great memory savings so I'm going to merge it. We can always revisit it later if we run into trouble.