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.