Bug Fixes:
- Fixed a recently introduced bug in posting list, which caused persisted postings to be skipped.
- Fix a long-standing bug, where we were dropping messages if `n.messages` channel was full. That caused loss of `MsgProp` messages, which had to be retried. There are other ways messages could be dropped, but doing it here was unnecessary.

Cleanup:
- Moved all tests to use docker-compose. test.sh now runs after running a 3-node cluster of Dgraph alphas.
- Added an internal retry mechanism for dropped proposals in both server and Zero.
- Moved random number generator to the common conn/node.go.
- Moved common Dgraph flags to RootCmd.
- Allowed Zero to expose its trace.
- All server package unit tests now use Docker-based Dgraph cluster, instead of spawning their own Dgraph instances.
- Query tests still work with their own Zero server.
- Upgrade Raft library.
- Improve tracking for proposal contexts.