Parallelize script verification

* During block verification (when parallelism is requested), script
  check actions are stored instead of being executed immediately.
* After every processed transactions, its signature actions are
  pushed to a CScriptCheckQueue, which maintains a queue and some
  synchronization mechanism.
* Two or more threads (if enabled) start processing elements from
  this queue,
* When the block connection code is finished processing transactions,
  it joins the worker pool until the queue is empty.

As cs_main is held the entire time, and all verification must be
finished before the block continues processing, this does not reach
the best possible performance. It is a less drastic change than
some more advanced mechanisms (like doing verification out-of-band
entirely, and rolling back blocks when a failure is detected).

The -par=N flag controls the number of threads (1-16). 0 means auto,
and is the default.

src/test/test_bitcoin.cpp

@@ -33,9 +33,13 @@ struct TestingSetup {
         pwalletMain = new CWallet("wallet.dat");
         pwalletMain->LoadWallet(fFirstRun);
         RegisterWallet(pwalletMain);
+        nScriptCheckThreads = 3;
+        for (int i=0; i < nScriptCheckThreads-1; i++)
+            NewThread(ThreadScriptCheck, NULL);
     }
     ~TestingSetup()
     {
+        ThreadScriptCheckQuit();
         delete pwalletMain;
         pwalletMain = NULL;
         delete pcoinsTip;