This fixes#857 and #1932. Also replaces PR #1725, since we don't need
our own implementation of condition_variable anymore since people who
update their polybar should have GCC 10 by now.
The m_updated_at field of the mpd module was removed instead of having
its clock change because it became unused in commit 645a3142a1.
Only updating when an mpd event occurred would cause issues when mpd was
playing and the machine was put to sleep because the elapsed time was
calculated by taking the time difference of the last update and now
which would give you wrong numbers, if the machine was in standby in
between.
Since the update function on the module is only called once a second (or
when an event happens), we can just update the data every time without a
huge performance hit.
Fixes#915
