Break away group zero into Dgraphzero.

This is a HUGE change.

Dgraphzero keeps track of membership information, and UID assignment.
Dgraphzero also uses Raft so we can have multiple instances working as a
Raft group. The lease and membership state is stored via Raft WAL.

Squashed commit of the following:
    Break away Raft node in conn package, so it can be shared by dgraphzero.

    Add dgraphzero code. Add tablet code.

    Remove uid range for now.

    Some more refactoring to push raft.Node into conn package.

    Dgraph zero started to crawl

    Working raft across dgraphzero.

    Ignore w directory

    Modify how membership is stored. Start working on proposals and contexts.

    Logic for proposing cluster joining and tablet assignment.

    Logic to handle membership updates.

    Store group zero as well.

    Port over lease code to dgraphzero as well.

    Start removing special group crap from worker package.

    Revert "Start removing special group crap from worker package."

    This reverts commit fea7bbc3b8c51e85b7d9071302039412c6329ed1.

    The whole thing finally compiles. Doesn't work, but compiles

    Remove some unneeded code.

    Janardhan's comments

    Add a TODO