lib: Make notmuch_database_get_directory return NULL if the directory is not found

Using the new support from _notmuch_directory_create, this makes
notmuch_database_get_directory a read-only operation that simply
returns the directory object if it exists or NULL otherwise.  This
also means that notmuch_database_get_directory can work on read-only
databases.

This change breaks the directory mtime workaround in notmuch-new.c by
fixing the exact issue it was working around.  This permits mtime
update races to prevent scans of changed directories, which
non-deterministically breaks a few tests.  The next patch fixes this.