rpc: have mintime account for timewarp rule

Previously in getblocktemplate only curtime took the timewarp rule into account.

Mining pool software could use either, though in general it should use curtime.

test/functional/mining_basic.py

@@ -153,6 +153,8 @@ class MiningTest(BitcoinTestFramework):
         # The template will have an adjusted timestamp, which we then modify
         tmpl = node.getblocktemplate(NORMAL_GBT_REQUEST_PARAMS)
         assert_greater_than_or_equal(tmpl['curtime'], t + MAX_FUTURE_BLOCK_TIME - MAX_TIMEWARP)
+        # mintime and curtime should match
+        assert_equal(tmpl['mintime'], tmpl['curtime'])
 
         block = CBlock()
         block.nVersion = tmpl["version"]