[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).
Before we had a keepalive that was switching states: wait->run->wait->run..., now we have one that will be just running, since we don't need to write the lastSyncTime anymore periodically(see removed ImapService::onUpdateLastSyncTime()), we don't need these intermediate states.