Rotate output enb.xlog ourselves at sync points so that nothing is lost
in the output (hello `logrotate copytruncate`) and so that we can emit
pre- and post- logrotate syncs.

Reuse logging's RotatingFileHandler and TimedRotatingFileHandler to
implement actual rotation, but carefully wrap them in our writer
classes so that we emit exactly the output we prepared explicitly
without any headers prepended by logging, and that we explicitly control
when rotation happens.

/proposed-for-review-at !5