Compress memcached logs post-rotation

We might be able to keep much more logs if we compress the files after rotation.

Local testing shows a 217.3 MiB memcached.log file can be deflate compressed to 7.0 MiB, a 1:30 compression ratio.

If we can get 150 days of logs, instead of 5 days, this could be useful for resolving issues in the future.

We will need changes to cbcollect_info to support compressed logs, but other than that it should be doable.