Because the wake pipe is thread-local, it was previously not possible
to wake an event loop across a thread. Therefore, this commit
rearchitects event loop waking by making the wake function a member of
the event loop itself and having it keep a pointer to its thread's wake
pipe. The global wake() function calls wake on the current thread's
event loop.
This also fixes a bug in BackgroundAction: it should wake the event loop
it was created on, instead of the current thread's event loop.
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