Wallet: store key creation time. Calculate whole-wallet birthday.

This also encapsulate wallet-read state information into CWalletScanState.
Wallet: optimize rescan to skip blocks prior to birthday
changes to thread code (directly use boost::thread)

- removes our NewThread() function an replaces remaining calls with
  boost::thread with our TraceThread template
- remove ExitThread() function
- fix THREAD_PRIORITY_ABOVE_NORMAL for non Windows OSes
Merge pull request #1863 from jgarzik/keytime

"Wallet birthdays": store key create time; calc whole-wallet birthday
Merge pull request #2553 from Diapolo/threads

changes to thread code (directly use boost::thread)