Time is not really relevant for us here and we don't use this timestamp in any
way. Yet it occupies 24 bytes and we do two clock_gettime calls to get it.
Replace it with blockStamp which is going to be used in the future for
transaction retransmissions.
It allows to improve single-node TPS by another 3%.
There is no such thing as high/low priority transactions, as there are
no free transactions anymore and they are ordered by fees contained
in transaction itself.
Closes#1063.
