Allow RocksDB to be shared across threads

Switch to boxed values for safety and add integration tests

Run rustfmt on the code.

Add Iterators and Snapshots

Improve safety of borrow checker with ffi objects

initial comparator

Fix a few memory leaks.  Add valgrind feature build.

Fix a few memory leaks.  Add valgrind feature for conditionally building a testable binary.  Get rid of unstable vector usage.

Add support for WriteBatch