Big ol' rewrite

[mappum]

This WIP PR contains the changes I have been working on for the past few months, which have over time replaced all parts of the existing codebase. While the tree algorithms have not changed and feature-wise it is in the same place as a few months ago, the internal types and interfaces have changed a lot and have resulted in much better code (the second time around I'm more experienced with Rust).

Main improvements:

• Caching/pruning - It is now possible to keep some or all tree nodes in memory across batches, meaning we don't have to fetch them from disk. This results in ~2.5x single-core performance increases if the entire tree can be held in memory, with the bottleneck then being disk writes.
• Succinctness - Comparable functions have been reduced by 50% LOC throughout the codebase.
• Maintainability - Saner abstractions in the internal interfaces, it will be much easier to change the code in the future.
• Performance - Compared to the previous benchmark of 18k updates per second, we can now get ~32k on a single core with <30MB memory usage (or 76k with the tree fully held in RAM) due to various optimizations. (These figures are on my 2017 MBP, with a 1M node tree).

I'll finalize this PR once the core Merk store API is finished, leaving it where the master branch currently is.

[davebryson]

This is looking good! One concern I have is opening rocksdb within Merk. This limits the ability to share the db connection across threads. For example, you may want that db handle available separately in the abci commit callback to save the latest root hash and height to the db for later use in abci info.

[mappum]

For example, you may want that db handle available separately in the abci commit callback to commit the latest root hash and height for later use in abci info.

That's a good point. I'm not necessarily set on the top-level API and it would be pretty easy to pass in a reference to a db handle in the Merk constructor, or have methods for raw access.

[mappum]

Merging these changes into develop because we're now at parity with what we had before the rewrite. Still needs a few things before a real release though.