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.
It seems some tools try to interpret the filename of message files,
even though they shouldn't:
"Do not try to extract information from unique names."
In particular, they seem to interpret the first part of the name (before
the first dot) as the # of seconds since the Unix epoch (ie.,
time(NULL)). That's not what mu/mu4e put there.
So, let's conform a bit more to the expected filename (as per the
maildir spec), so we're not confusing those tools.
(since N (new) messages cannot have any other flags, you would loose
e.g. the T flag when moving to trash; now, we remove the N flag, and the T
flag remains)