BP-47: Support for writing ledger entries with O_DIRECT

**BP**

> Follow the instructions at http://bookkeeper.apache.org/community/bookkeeper_proposals/ to create a proposal.

This is the master ticket for tracking BP-47 :

Add support for writing ledger entries, bypassing the file system's buffering. On supported systems (currently Linux and MacOS), files are opened with O_DIRECT and page-aligned buffers are maintained by the BookKeeper application. Access to the operating system's IO API is via a thin JNI binding.

Proposal PR - https://github.com/apache/bookkeeper/pull/2932