Review feedback from @dangell7 and @mvadari

- Rewrite isTesSuccess to use TERSubset::operator bool
- Add Transactor::preflightSigValidated for expensive operations that
  should be done after signature validation. These things would have
  been done after preflight2 before this refactor.
- Split Batch and EscrowFinish preflight to use preflightSigValidated, too.

include/xrpl/protocol/TER.h

@@ -673,7 +673,8 @@ isTerRetry(TER x) noexcept
 inline bool
 isTesSuccess(TER x) noexcept
 {
-    return (x == tesSUCCESS);
+    // Makes use of TERSubset::operator bool()
+    return !(x);
 }
 
 inline bool