don't record transactions for Time* fields on merge

1) merge shouldn't generate any transactions except merge
2) recording Time* change transaction mess numbers in history.
   As target ticket gets all transactions from both sides then
   history is already correct. Yes, numbers in the history are
   strange, for example you can have two txns saying time worked
   was changed from 0 to X and from 0 to Y, but the difference
   is what's important and is correct.
script that deletes bad (Set, TimeWorked) txns

It try to fix history and do it as safe as possible.
It calcs time worked using history, by the way collects
candidates for deletion. Transactions are deleted only
if time on ticket is equal to time from history minus
time on candidates.

It's probably possible to write similar script for TimeLeft
and TimeEstimated, but it's much harder to do as the value
is not recorded on Create unlike time worked which is stored
in TimeTaken column.
update upgrade docs with manual step
Merge branch '4.2/time-fields-on-merge'