Rework the clients thread priority handling

- removes SetThreadPriority() and integrates that into NewThread() with a
  default of THREAD_PRIORITY_NORMAL
- removes special-casing (priority switching) for internal Bitcoin miner
- uses a new default for the following threads: ThreadScriptCheck (below
  normal - because normal prio threads on every CPU core could slowdown UX),
  ThreadImport (above normal - to speed it up a little) and
  ThreadBitcoinMiner (below normal - to compensate the removed special
  casing)
- fixes THREAD_PRIORITY_ABOVE_NORMAL for non Windows OSes as normal and
  above normal were the same before