Add an option --lazy-check to ignore any directories that don't have
their ctime changed since the last indexing operation.
There are a few corner-cases (such as editing a message outside mu's
control) where this might miss a change, but apart from that, makes
indexing in for a maildir (and its sub-maildirs) almost a no-op if there
were no changes.
Seems people are getting really big mails these days, so let's up the
default (which is also what mu4e uses) to 500 Mb (which should be enough
for everyone, always)
