Tube recvuntil str find bufsize fastpath2

Restores the performance of `tubes` to near what they were before.  Not quite as high, but much better than `2.1.3` (~200kbps) and ten times as fast as the current `master` (~24000kbps).

Additionally actually enforces timeout changes when `Timeout.local` is entered an exited, which should have always been implemented.

Fixes #238.