Merge branch 'mjog/imap-connection-fixes' into 'gnome-3-36'

IMAP connection fixes

See merge request !479 (merged)

(cherry picked from commit 297a59ca)

6d229501 Add a simple mock server for testing network code
95c1916a Add unit test for Geary.Imap.ClientConnection
339d8ae2 Add unit test for Geary.Imap.ClientSession
d2724e1c Remove last vestiges of TCP graceful disconnect from IMAP code
cc5efc5f Minor doc comment update
7e77133b Update Geary.Imap.Command API
62f1df12 Rework Geary.Imap.ClientConnection signal and error handling
67e9a4cc Update Geary.Imap.Deserializer
e0b973be Update Geary.Imap.Deserialiser implementation
79ba094f Fix Geary.Imap.Command never receiving a response timeout
cd924e55 Clean up Geary.Imap.ClientSession disconnect handling
57a1ef3f Fix Geary.Imap.ClientSession::disconnect_async not working
ca7cc04a Remove Geary.Imap.ClientSession lifecyle signals
54156003 Add TestCase.assert_double
13d43d41 Update Geary.Imap.ClientSession connect timeout handling
53ce727c Minor fixes
b46838f1 Update Geary.Imap.Capabilities handling
7ac72379 Update Geary.Imap.ClientSession namespace handling
9e01d8dc Remove Geary.Imap.ClientSession::server-data-received signal
b53d5b24 Simply Geary.Imap.ClientService selected mailbox handling
31260ab1 Fix Geary.Imap.ClientService sometimes not closing sessions
73474513 Geary.Imap.Serialiser: Stop using GDataOutputStream
52a9102f Geary.Imap.Serializer: Trivial doc comment update
47945cbd Geary.Imap.ClientConnection: Simplify serisaliser buf handling
79125d1c Geary.Imap.Serializer: Remove unused id and to_string members