Single-threaded is the build-default, and seems to work well enough for
1.12.7, so remove the option to turn it off.
This is because build-options that influence such low-level/core
behavior are a pain to maintain.
In lazy-mode, we were skipping directories that did not change; however,
this didn't help for the case were users received new messages in big
maildirs.
So, add another check where we compare the ctime of message files with
the time of the last indexing operation. If it's smaller, ignore the
message-file. This is faster than having to consult the Xapian database
for each message.
Note that this requires in mu4e:
(setq mu4e-index-lazy-check t)
or
--lazy-check
as a parameter for 'mu index'.
