Introduce library-independent default compression level

Introduce library-independent default compression level

Summary:
Previously we were using -1 as the default for every library, which was legacy from our zlib options. That worked for a while, but after zstd introduced https://github.com/facebook/zstd/commit/a146ee04ae5866b948be0c1911418e0436d80cb4, it started giving poor compression ratios by default in zstd.

This PR adds a constant to RocksDB public API, `CompressionOptions::kDefaultCompressionLevel`, which will get translated to the default value specific to the compression library being used in "util/compression.h". The constant uses a number that appears to be larger than any library's maximum compression level.
Closes https://github.com/facebook/rocksdb/pull/3895

Differential Revision: D8125780

Pulled By: ajkr

fbshipit-source-id: 2db157a89118cd4f94577c2f4a0a5ff31c8391c6