zebra-state: replace sled with rocksdb

[yaahc]

Motivation

Prior to this PR we've been using sled as our database for storing persistent chain data on the disk between boots. We picked sled over rocksdb to minimize our c++ dependencies despite it being a less mature codebase. The theory was if it worked well enough we'd prefer to have a pure rust codebase, but if we ever ran into problems we knew we could easily swap it out with rocksdb.

Well, we ran into problems. Sled's memory usage was particularly high, and it seemed to be leaking memory. On top of all that, the performance for writes was pretty poor, causing us to become bottle-necked on sled instead of the network.

Solution

This PR replaces sled with rocksdb. We've seen a 10x improvement in memory usage out of the box, no more leaking, and much better write performance. With this change writing chain data to disk is no longer a limiting factor in how quickly we can sync the chain.

The code in this pull request has:

• Documentation Comments
• Unit Tests and Property Tests

Review

@hdevalence

[dconnolly]

😬 on debian-buster

[yaahc]

grimacing on debian-buster

oh joy, more c++ dependency management

[hdevalence]

LGTM!

One thing we'll want to do later is fiddle with the RocksDB options; using the defaults as in this PR is a good start until we do so.

Also, we previously dropped the async state accesses because of sled issues, but we should consider bringing that design back at a later point (this is a performance optimization, so let's aim for functional completeness first).