Caused by a revert accident in commit
f2cdbeac

Closes https://github.com/MusicPlayerDaemon/MPD/issues/631

This reverts commit 58d7804d.  It
caused a use-after-free bug when Client::OnSocketError() was called
due to a failed write, e.g. if the output buffer was full.

release v0.21.14

The new Response instance in the `catch` block didn't have the
`command` attribute set, so the error response didn't indicate which
command had failed, which however is required in the MPD protocol.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/628

Clarify that those can't throw, preparing for the next commit.

Previously, a bogus value (whatever happened to be still in
`start_time`) was used.

See code comment.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/629

This optimization is useless because sane pthread_cond_signal()
implementations check the number of waiters and do not invoke a system
call if there are none.

And allow "Z" suffix after date.

Prepare for adding more flexible parsing.

Move code from Parser.cxx.

This is recommended by the strptime() manpage, because strptime() does
not initialize/set attributes which were not specified in the format
string.

Simplify the plugin by using a trivial relative URI; later,
playlist_check_translate_song() will set the correct full URI.