Enforce a minimum name coin amount.

Introduce a softfork that enforces a minimum amount of locked coins for
each (newly updated) name.
Reject tx with too low fee from mempool also before fork.

For inclusion into mempool and newly mined blocks, reject name
transactions with too little locked coins even before the fork point.
Merge pull request #205 from domob1812/enforced-fee

Enforce locked name coin amount.