lib: Make directory document creation optional for _notmuch_directory_create

Previously this function would create directory documents if they
didn't exist.  As a result, it could only be used on writable
databases.  This adds an argument to make creation optional and to
make this function work on read-only databases.  We use a flag
argument to avoid a bare boolean and to permit future expansion.

Both callers have been updated, but currently retain the old behavior.
We'll take advantage of the new argument in the following patches.