PHP-953: Check full NS length.

Can we leave a comment here that 3 is for the two periods and a dollar sign? The function `get_namespace_length()` is generically named and I wouldn't expect this to add a trailing `.$` -- after all, the namespace of a collection is simply `db.collection`. Alternatively, just add 1 in this function and then add the extra 2 for `.$` in the command/legacy functions below (with a comment that it's for period and dollar sign).