Fixes regression from commit 31bad5f7:
if Pause() returns true, the output thread is running a busy loop,
causing 100% CPU usage, instead of just closing the output.

Closes #105

The intrusive contains can easily erase items without searching
through the whole list.  This removes a good amount of runtime
overhead.

By using an "intrusive" data structure, we can easily eliminate
struct TimerRecord.

Prepare to eliminate the TimerRecord struct.

Got lost in commit e5ee3579

We can do CURL requests asynchronously, and we don't need a
synchronous WorkQueue thread for that.

This allows parallelizing lookups and allows immediate cancellation.

Allows database plugins to use the EventThread, e.g. for CURL
integration.

release v0.20.10

This fixes an assertion failure caused by resuming playback before the
decoder has finished startup.

contains OSX mixer

memcmp() can overflow the buffer.

Allows eliminating lots of complexity and workarounds for bogus
assertion failures.