log start and end of Shutdown()

- could be helpful when debugging shutdown related problems

Rebased-by:   Warren Togami <wtogami@gmail.com>
Rebased-from: ced3c248168941fbbd42d5a3807657a88be6a54e

https://github.com/bitcoin/bitcoin/pull/3038
Based on this bitcoin master commit.

The MacOS X corruption is suspected to be shutdown related.  We also currently experience a non-fatal shutdown error on Linux with boost-1.53.  So there are multiple reasons why this could be useful to debug.