[qmf] Introduce QMailAccount::AlwaysOnMode status flag

Use QMailAccount::AlwaysOnMode 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).
Why change from 'backgroundActivity->wait()' to 'backgroundActivity->run()' here?