Improve Java API get() performance by reducing copies (#10970)

Improve Java API get() performance by reducing copies (#10970)

Summary:
Performance improvements for `get()` paths in the RocksJava API (JNI).
Document describing the performance results.

Replace uses of the legacy `DB::Get()` method wrapper returning data in a `std::string` with direct calls to `DB::Get()` passing a pinnable slice to receive this data. Copying from a pinned slice direct to the destination java byte array, without going via an intervening std::string, is a major performance gain for this code path.

Note that this gain only comes where `DB::Get()` is able to return a pinned buffer; where it has to copy into the buffer owned by the slice, there is still the intervening copy and no performance gain. It may be possible to address this case too, but it is not trivial.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10970

Reviewed By: pdillinger

Differential Revision: D42125567

Pulled By: ajkr

fbshipit-source-id: b7a4df7523b0420cadb1e9b6c7da3ec030a8da34