Made expicit constructor CTransaction(const CMutableTransaction &tx).

This makes the above constructor explicit. The rationale is that this conversion has very significant performance effects. Making it explicit makes it easier to reason about these performance trade-offs, and helps identify possible functions that need a CMutableTransaction version.

src/primitives/transaction.h

@@ -298,7 +298,7 @@ public:
     CTransaction();
 
     /** Convert a CMutableTransaction into a CTransaction. */
-    CTransaction(const CMutableTransaction &tx);
+    explicit CTransaction(const CMutableTransaction &tx);
     CTransaction(CMutableTransaction &&tx);
 
     template <typename Stream>