wal: remove old xlog files asynchronously

In contrast to TX thread, WAL thread performs garbage collection
synchronously, blocking all concurrent writes. We expected file removal
to happen instantly so we didn't bother to offload this job to eio
threads. However, it turned out that sometimes removal of a single xlog
file can take 50 or even 100 ms. If there are a dozen files to be
removed, this means a second delay and 'too long WAL write' warnings.

To fix this issue, let's make WAL garbage collection fully asynchronous.
Simply submit a jobs to eio and assume it will successfully complete
sooner or later.  This means that if unlink() fails for some reason, we
will log an error and never retry file removal until the server is
restarted. Not a big deal. We can live with it assuming unlink() doesn't
normally fail.

Closes #3938