Remove Bunny::Queue#pop_waiting

A little bit of background: on MRI, the method raised `ThreadErrors`
reliably. On JRuby, we used a different [internal] queue implementation
from JDK so it wasn't an issue.

`Timeout.timeout` uses `Thread#kill` and `Thread#join`, both of which
eventually attempt to acquire a mutex used by Queue#pop, which Bunny
currently uses for continuations. The mutex is already has an owner
and so a ThreadError is raised.

This is not a problem on JRuby because there we don't use Ruby's
Timeout and Queue and instead rely on a JDK concurrency primitive
which provides "poll with a timeout".

Finding a workaround will probably take a bit of time and may involve
reimplementing standard library and core classes.

We don't want this issue to block Bunny 0.9 release. Neither we want
to ship a broken feature.  So as a result, we will drop
Bunny::Queue#pop_waiting since it cannot be reliably implemented in a
reasonable amount of time on MRI.

Closes #131.