new: read db_files and db_subdirs only if mtime changed

new: read db_files and db_subdirs only if mtime changed

The db_files and db_subdirs are unnecessary for unchanged directories.

maildir with 10000 e-mails:

old version:
	$ time ./notmuch new
	No new mail.

	real    0m0.053s
	user    0m0.028s
	sys     0m0.026s

new version:
	$ time ./notmuch new
	No new mail.

	real    0m0.032s
	user    0m0.009s
	sys     0m0.023s

Signed-off-by: Karel Zak <kzak@redhat.com>
Reviewed-by:  Austin Clements <amdragon@mit.edu>
Looks good (faster than, but provably equivalent to the original code!
notmuch_directory_get_child_* are side-effect free,
db_files/db_subdirs aren't used between where they were set in the old
code and where they are set in the new code, and db_files/db_subdirs
are initialized to NULL when declared).

Another timing data point:
Old code: ./notmuch new  0.77s user 0.28s system 99% cpu 1.051 total
New code: ./notmuch new  0.09s user 0.27s system 98% cpu 0.368 total