Remove contention on signature cache during block validation

Since block validation happens in parallel, multiple threads may be
accessing the signature cache simultaneously. To prevent contention:
* Turn the signature cache lock into a shared mutex
* Make reading from the cache only acquire a shared lock
* Let block validations not store their results in the cache

src/script.h

@@ -32,6 +32,7 @@ enum
     SCRIPT_VERIFY_NONE      = 0,
     SCRIPT_VERIFY_P2SH      = (1U << 0),
     SCRIPT_VERIFY_STRICTENC = (1U << 1),
+    SCRIPT_VERIFY_NOCACHE   = (1U << 2),
 };
 
 enum txnouttype