Add a feature to require minimum compression level

[cowtowncoder]

Currently any amount of compression, no matter how small, will result in compression being used. But due to cost of uncompression, it may not make sense to store compressed content for negligible amounts. So: it would make sense to add a configuration value to specify maximum rate of content after compression (default could be something like 0.95 to require minimum of 5% compression) that will still be stored as compresed.

Note that checking may be inexact, due to limitations wrt streaming operations: that is, it is acceptable to use heuristics to estimate total compression, for example by compressing first N bytes of content.