Rework block processing benchmark code

* Replace -benchmark (and the related fBenchmark) with a regular debug option, -debug=bench.
* Increase coverage and granularity of individual block processing steps.
* Add cummulative times.

src/main.h

@@ -91,7 +91,6 @@ extern CWaitableCriticalSection csBestBlock;
 extern CConditionVariable cvBlockChange;
 extern bool fImporting;
 extern bool fReindex;
-extern bool fBenchmark;
 extern int nScriptCheckThreads;
 extern bool fTxIndex;
 extern bool fIsBareMultisigStd;