Auto merge of #1772 - RalfJung:less-timeout-checking, r=RalfJung

only check timeouts when a thread yields

Currently, we check for expired timeouts after each step of execution. That seems excessive. This changes the scheduler to only check for timeouts when the active thread cannot continue running any more.

`@vakaras` does this sound right? `pthread_cond_timedwait` anyway already yields, of course, since it blocks on getting the signal (or the timeout).

diff --cc src/thread.rs
Simple merge