In-memory datastore for aleph peers

[yusefnapora]

This adds a Datastore class that's backed by the in-memory MemDOWN backend for LevelUP, the node leveldb library.

I went this route rather than rolling my own in-memory kv store because

• this gives us an easy path to a persistent datastore by using the leveldb (or rocksdb) backend
• it forced me to use an async put/get interface, which we'll need if we do the above
• it was pretty easy

Aleph nodes now supports the /mediachain/data protocol to fetch data, but writing to the datastore needs to happen in-process (no push or merge support yet). Still, merge should be fairly easy to accomplish, since we've got remote query working already.

I added a simple test between two aleph nodes, but haven't done an integration test yet.

[vyzo]

:+1: for having a real datastore.

Basically, in terms of workflow, aleph can be the workspace where you are building your data, and when you are ready to publish you push them to your concat.