When mining the first block of a new signet chain, pick a timestamp for
the first block so that after mining 100 blocks the timestamp will be
back to the current time -- this prevents an unnecessary delay before
any miner rewards have matured enough to be spent. This takes into
account that the delta between blocks may be shorter than 10 minutes due
to attempting to increase the difficulty to match --nbits, but does not
take into account the time spent actually generating the 100 blocks.
