Optimize uncompressed blobs

[crepererum]

Is your feature request related to a problem? Please describe.
Currently setting "no compression" for the blobs still sets a custom compressor for value-log:

lsm-tree/src/blob_tree/mod.rs

Lines 84 to 87 in f04590d

	let vlog_cfg =
	value_log::Config::<MyBlobCache, MyCompressor>::new(MyBlobCache(config.cache.clone()))
	.segment_size_bytes(config.blob_file_target_size)
	.compression(MyCompressor(config.blob_compression));

This then skips this optimization here:

https://github.com/fjall-rs/value-log/blob/6f84890727c6f5906619d32a0886fa0c85eeea61/src/segment/reader.rs#L94-L106

but instead needs to clone the data unnecessarily often (here and here). I think at least one of these memcpys is avoidable.

Describe the solution you'd like
I think the cleanest way would be to set the value-log compressor to None.

Describe alternatives you've considered
Pass the decompress value owned instead of sliced/borrowed.

Additional context
-

[marvin-j97]

Good catch. Should be easy to fix as you described. Though I wouldn't really recommend no compression on blobs unless data is really high entropy.

[crepererum]

Though I wouldn't really recommend no compression on blobs unless data is really high entropy.

Which includes my use case: large binary blobs of data that have their own compression already applied, so compressing them again w/ a general purpose compressor is somewhat pointless.

[marvin-j97]

Okay damn well looking at this, the problem is that the compression starts off as CompressionType::default(), and the compression is not Option<CompressionType>, but CompressionType.

So solving this probably requires changing the trait in value-log.

[marvin-j97]

WIP @ fjall-rs/value-log#30