Add MyRocks into MariaDB repository

How do we add MyRocks into MariaDB, and how do we merge it?

In the facebook/mysql tree it is done as follows:

• /rocksdb - a submodule pointing to rocksdb repo
  *storage/rocksdb - MyRocks code.

Results of discussions with serg:

• We can put the storage engine into storage/rocksdb
• The preferered place to put the submodule is {{ storage/rocksdb/rocksdb}}.

• Merging *
  Sregei uses a technique called "Merge Tree" for TokuDB merges.
  The idea is to create a tree on github where you re-commit the newer versions.

cp tokudb-tree/*.cc  mariadb-merge-tree/

git commit -m "TokuDB version X.Y.Z"

cd mariadb-tree

git pull --merge mariadb-merge-tree

This destroys the revision history in MyRocks. But this is the best thing we can get it seems.