remove --writer-threads option
Support for multiple writer threads was broken, and profiling had shown
it to be of dubious utility.
https://github.com/internetarchive/warcprox/issues/101
https://github.com/internetarchive/warcprox/wiki/benchmarking-number-of-threads
and
WarcWriterProcessor.close_for_prefix()
New API to allow some code from outside of warcprox proper (in a
third-party plugin for example) can close open warcs promptly when it
knows they are finished.
and