Implement a new message indexer consisting of a single-threaded scanner
and a multi-threaded indexer.
This allows for a number of optimizations as well as background
indexing, though this initial version should be behave similar to the
old indexer.
reimplement the old mu-log.[ch] into mu-logging.{cc,hh}
If available (and using an appropriately equipped glib), log to the
systemd journal
Only g_criticals have stderr output, all the other g_* go to the log
file / journal.
