Massively improve secure sync speed (divide time spent by 10)
* Also cleanup some useless code but we need more cleanups
Another massive sync performance improvement.
* Check mailbox validity before cursor iterations, 3 tests * msgNumber removed
* Build messageIDs string only for current checked mailbox
* Don't call push HTTP method if there is no messages to sync
Label fix, 0.16 tagging for Play Store and slow sync mode commented.