updated dependencies, fixed race condition

[bergos]

This PR fixes some issues and updates all dependencies to the latest major versions.

Fixes and updates:

• The command method of the Raft class tried to use a non existing function to forward an error for the case that it was called without being the leader. This was replaced by a standard throw with the address of the leader attached to the error object.
• The latest version of levelup no longer supports string values for keyEncoding. Very simple helper functions are used to convert the integer key values to hex strings.
• The commandAck method of the Raft class contained a race condition problem. The problem mainly shows up with a bigger number of nodes. A promise queue is used to solve the problem.
• The latest version of mkdirp uses an Promise API. The util.promisify wrapper was therefore removed.

[kakserpom]

@bergos, you are amazing! 👍