[wallet] Consider generated coins mature at COINBASE_MATURITY+1

We're not seeing large reorgs that would justify waiting a large
 amount past the rule required maturity, and the extra three
 hours is just a nuisance. Take one more block to at least give
 the 100th block time to propagate.
Merge pull request #2947 from gmaxwell/theyre_maturing_faster_these_days

[wallet] Consider generated coins mature at COINBASE_MATURITY+1