[mms] Add Horde_Stream#close().

Can't automatically close in destruct(), since we allow public access to
the stream resource and that resource may still be used somewhere.

For 2.0, should make stream a private member variable and require access
via the Horde_Stream API instead.
This fixes DB tests for me

Mysql tests were trying to be run for me, even though I don't have mysql
on my system.