Make CMutableTransaction constructor explicit

Silently converting to a CMutableTransaction will drop all caches
and should thus be done explicitly

src/test/txvalidationcache_tests.cpp

@@ -24,7 +24,7 @@ bool CheckInputs(const CTransaction& tx, CValidationState &state, const CCoinsVi
 BOOST_AUTO_TEST_SUITE(tx_validationcache_tests)
 
 static bool
-ToMemPool(CMutableTransaction& tx)
+ToMemPool(const CMutableTransaction& tx)
 {
     LOCK(cs_main);