Each account had one mailbox per folder, which had one associated
collection. Now each Account has one Collection for all folders and each
Mailbox object holds only the hashes of each message.
Collection also gets Threads for each folder in order to mix messages
(ie from/to Sent folder).
Insert Sent emails in chronological order
if inserted unsorted, mails a, b with a happened-before b, might never
get added.
Fix multiple insertions in ThreadTree upon insert_reply
insert_reply was creating multiple copies in threading
snooze field translated to whether the user wants to be notified of new
email in the thread or not. This is toggled with an (temp) EX cmd: `toggle_thread_snooze`
closes#112