Add logging for LZ4Factory instance type

[jon-wei]

The LZ4Factory instance used for compression can have three different implementations:

• native
• unsafe
• safe

The implementation used can affect the size of compressed output, so this PR adds additional logging around the LZ4 library to show what instance is being used. The value of java.library.path is also logged, since the native instance depends on libc > 2.14 being available on that path.

This PR has:

• been self-reviewed.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• been tested in a test Druid cluster.