[qmf] Introduce QMailAccount::HasPersistentConnection status flag

Use QMailAccount::HasPersistentConnection status flag for IMAP idle instead of
updating last sync time every minute, this reduces accounts db writes(they
trigger notifications to other processes) and also make use of a linger
keepalive(less state transitions).
Merge pull request #60 from nemomobile-packages/alwaysOn

[qmf] Introduce QMailAccount::AlwaysOnMode status flag