Add new variants for "compress only if comp rate at least N"

[cowtowncoder]

A relatively common use case for my own use is one where compression is only used if it makes significant difference. For example, if compression rate is only 2%, it hardly makes sense to store compressed version as there is still additional decompression overhead when reading content.

While caller can verify compression rate and choose to discard content in such case, it would be bit more efficient if library had a method that could avoid allocating result buffer in such cases. So let's add a variant or two that allows such "opportunistic" compression.

If possible, this should be added both for LZF and convenience Deflate (gzip) compression methods.

[cowtowncoder]

Forgot that the only shared abstraction is Uncompressor, and that there is no block-based compressor for gzip.

So ended up just adding ChunkEncoder.appendEncodedIfCompresses(...).