Fix yamux handling of writes bigger than the window size

[ianopolous]

Also fix inefficient window size handling.

I tried to test this with ping, but of course ping doesn't handle messages sent in multiple parts. But we have tested this downstream.

[mxinden]

In case it is of some use, in rust-yamux we split data frames at 16* 1024 bytes.

By limitting the data frame payload size one gains the following
beneftis:

1. Reduce delays sending time-sensitive frames, e.g. window updates.

2. Minimize head-of-line blocking across streams.

3. Enable better interleaving of send and receive operations, as each is
   carried out atomically instead of concurrently with its respective
   counterpart.

Limiting the frame size to 16KiB does not introduce a large overhead. A
Yamux frame header is 12 bytes large, thus this change introduces an
overhead of ~0.07%.

See libp2p/rust-yamux#111 for details.

[Nashatyrev]

LGTM