WIP; Add new rocksdb ChainDB implementation as alternative to Hammersbald

[notmandatory]

This PR needs to be rebased after #40 , #41 and #42 are merged.

[prusnak]

Can you please describe what are the advantages and disadvantages of hammersbald and rocksdb? That is I am interested in hearing 4 sides of the story (HB+, HB-, RDB+, RDB-).

[notmandatory]

@prusnak my initial motivation for this change was that I ran into problems with hammersbald when I used it in a mobile (android) app and I wanted to see how a more common (for mobile) embedded DB would perform. My understanding is that hammersbald's primary advantages is better performance, which may be important for a multi-user server app but I believe is less so for a single user mobile app. In my (non-exhaustive) initial test both DBs had equivalent performance loading all headers (646k) from storage to memory cache. Both DBs are also similar in size and dependencies, but RDB is clearly the winner in terms of maturity.

Feature	hammersbald	rust-rocksdb	Note
Maturity	-	+	contrib/forks: HB 3/11, RDB 97/318
Dependencies	+	+	HB 1 (hex) , RDB 2 (libc, rocksdb-sys)
Size	+	+	HB 26.1 kb, RDB 79.6 kb
Multi-threaded	+	+
Performance	+	+	quick test on macbook, both ~5-6 sec to load header cache

[notmandatory]

I've put this on the back burner right now, but if others are interested I'm happy to get it ready for actual review and merging.

[notmandatory]

I don't think I'll be getting back to this one so closing to get it off anyone's radar.