Event notifier from golang to python

[mhchia]

What was wrong?

• Content validation should be done in Python
• Lack of interface for Python to control go code
  • https://github.com/ethresearch/sharding-p2p-poc/issues/16
• Lack of refactoring

How was it fixed?

• Added EventNotifier(currently done through gRPC). It send data to Python side from Golang, and thus Python side send the response back to Golang
  • Currently use gRPC, which can be replaced by IPC in the future
  • NotifyCollation(collation) returns bool: send new received collations from Golang side to Python, and get the validation result of the collation: isValid
  • GetCollation(shardID, period, collationHash) returns collation: send "collation request" from Golang to Python side, and ask for collations from Python side.
• Used Validator for content validation
  • Fixed https://github.com/ethresearch/sharding-p2p-poc/issues/18
  • Set a Validator function in pubsub for collations' topics. This Validator simply sends collations from Golang to Python through EventNotifier.NotifyCollation, the collation is validated in Python side, and send the validation result from Python side back to Golang
• Added RPC SendCollation for Python side to send blocks
• gRPC server and client code in Python
• Refactored ShardPrefTable with added locks to avoid race condition: https://github.com/ethresearch/sharding-p2p-poc/issues/57

TODOs

• [x] Rebase
• [x] Add getCollation to eventNotifier
• [x] Add test for eventNotifier
• [x] remove unnecessary comments
• [x] Rename and refactor ShardPrefTable

Cute Animal Picture

[mhchia]

Reverted the version update of some deps in https://github.com/ethresearch/sharding-p2p-poc/pull/54/commits/576dd2278c644567b99ff51da46670e09e33bd82, which should be solved in other PR Tracked in https://github.com/ethresearch/sharding-p2p-poc/issues/55

[mhchia]

Ready for review

[mhchia]

Note: TODO: add documents for "how to communicate with shardingp2p nodes and receive notification from them"