The normal I/O event thread can have a large latency, e.g. when
libgnutls loads all TLS CA certificates for a https connect.  This
makes it unreliable for the ALSA I/O notifications, and causes ring
buffer xruns.  To avoid interfering with high latency events such as
CURL's, we move the ALSA I/O events to a separate I/O thread which
also obtains real-time scheduling (if possible).

Closes #221

Works around a deadlock bug which happens when waiting for an
EventLoop which isn't running anymore.

Fixes thread sanitizer warnings.

Fixes thread sanitizer warnings.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Move its code to DeferEvent instead.

Passing it by value is actually smaller (32 bit) than the rvalue
reference (64 bit pointer), and it ensures that the object is consumed
after the call returns, no matter how the methods are implemented.

Migrate callers to GetSocket().Read(), GetSocket.Write(), which is the
same.