SERVER-7965 Make the timestamp format settable in servers.

From the command line or config file, --logTimestampFormat=(ctime|iso8601-utc|iso8601-local).

The beginning of the Unix Epoch is formatted as follows in the three formats,
assuming America/New_York for the time zone.

  ctime:         Wed Dec 31 19:00:00.000
  iso8601-utc:   1970-01-01T00:00:00.000Z
  iso8601-local: 1969-12-31T19:00:00.000+0500

The default remains the same as in prior versions, "ctime".