Using kt_log (syslog) in addition to stderr to report configuration failures

When running via upstart, kafkatee's output is redirected either to
/dev/null or into /var/log/upstart/kafkatee.log.  Since we expect
other kafkatee messages to go to syslog, it is difficult to locate
error messages due to bad configuration if those logs do not also
go to syslog.

Also added log messages for starting and exiting in main thread.

Change-Id: I23fd7e5e377cc5670bb589da0e3a1d836a8699f1