This patch solves the following memory/resource leak problems:

(a) Gee.TreeSet doesn't drop references when destroyed.  Fixed by
using a subclass that clears the set when destroyed (exactly same
as patch made to Gee, however that won't be in distribution for
awhile.)

(b) Imap.ClientSession was holding refs to CommandResponses after
they'd been completed.  They're now dropped.

(c) Imap.ClientSessionManager now has an open/close_async() (called
by Imap.Account.open/close_async()) that drops all ClientSessions.

(d) All classes in Engine (and some in the client) use Geary.BaseObject,
which uses a static map to track outstanding held references to
it.  The table is dumped when Geary exits.  Must be enabled with a
./configure flag.

Two outstanding memory leaks persist (one for Imap.ResponseCodes and
another when messages are selected/deselected), so this doesn't close
the ticket, but testing and use has shown these changes to make a huge
improvement on me...