Tracking issue for project re-organization and cleanup

[poga]

This is a tracking issue for the on-going project re-organization and cleanup.

Steps:

• [ ] Migrate to Go modules #220
• [x] Extract standalone ptt.ai-core module. #244
• [ ] end-to-end test for ptt.ai-core
• [ ] API document for ptt.ai-core. Goal: pass go-lint
• [ ] replace bundled log module with upstream, possibly a fork
• [ ] Document HTTP and RPC API
• [x] Clarify license #251
• [x] extract crypto package #266
• [x] extract rlp package #267
• [x] extract event package #267
• [x] extract metric package #267

Related issues:

• [ ] Remove ethereum p2p. #216
• [x] Separate raft as the independent repo forked from etcd-io/etcd #250

Unresolved questions: