Enable CHECKLOCKTIMEVERIFY as a standard script verify flag

Transactions that fail CLTV verification will be rejected from the
mempool, making it easy to test the feature. However blocks containing
"invalid" CLTV-using transactions will still be accepted; this is *not*
the soft-fork required to actually enable CLTV for production use.

Rebased-From: ffd75adce01a78b3461b3ff05bcc2b530a9ce994

src/script/standard.h

@@ -51,6 +51,7 @@ static const unsigned int STANDARD_SCRIPT_VERIFY_FLAGS = MANDATORY_SCRIPT_VERIFY
                                                          SCRIPT_VERIFY_NULLDUMMY |
                                                          SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS |
                                                          SCRIPT_VERIFY_CLEANSTACK |
+                                                         SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY |
                                                          SCRIPT_VERIFY_LOW_S;
 
 /** For convenience, standard but not mandatory verify flags. */