Implement SocketDB

[cwgoes]

From conversation with @jaekwon -

jaekwon - btw we also need to create a socket version of DB so we can connect an external DB… SocketDB.
maybe using GRPC or something.  lmk if you have time to work on it.

cwgoes
I can probably work on SocketDB next week, did you have a particular gRPC interface in mind / should SocketDB include "socketDB server" which interfaces to a local non-socket DB?

jaekwon
yes, and we just need to support CLevelDB there for now.
i don’t have a particular gRPC interface in mind.  I wonder if someone’s already done this work.
it also needs security so we need to be able to do a TLS handshake first… or use secret_connection.

[ebuchman]

Looks like Odeke started working on this in #162.

Theres also already an issue for this in tendermint/tendermint#1162

I suppose we can close that one and let this one track.

[odeke-em]

@ebuchman in deed :)

Please feel free to take over it @cwgoes and @ebuchman: my PR allows for authentication and it is over gRPC both with single and bidirectional streaming -- I believe it satisfies all the requested requirements from @jaekwon and @ebuchman, but perhaps could even be tweaked further

[cwgoes]

To be added: secure transport layer (helpful guide).

[cwgoes]

Do we want batch operations? Presumably...

Also, do we want any kind of automatic certificate generation, or is it fine to delegate that to library users?

[cwgoes]

Closed by #162. Still going to attempt CLevelDB deterministic builds.