MessageBus: immutable contracts

[ChiTimesChi]

Description

This PR aims to provide an immutable MessageBus, which is supposed to serve as a backbone for the messaging infra on all the chains. All potential upgradeable logic is separated into separate contracts:

• GasFeePricing is responsible for calculating the fees for sending any type of message.
• AuthVerifier is responsible for verifying the submitted messages, which can be later upgraded to verifying messages using submitted proof.
• MessageExecutor is responsible for executing messages of any types.

MessageBus is responsible for:

• Sending messages:
  • Applying correct fee from GasFeePricing.
  • Emitting events, indicating that message was sent.
• Receiving messages:
  • Checking that every message can be executed only once.
  • Checking that message sender is authenticated by AuthVerifier.
  • Passing message correctly to have it executed by MessageExecutor.

Checklist

• New Contracts have been tested
• Lint has been run
• I have checked my code and corrected any misspellings

[coveralls]

Pull Request Test Coverage Report for Build 2391070410

• 0 of 47 (0.0%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 26.314%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
contracts/messaging/ContextChainId.sol	0	2	0.0%
contracts/messaging/HarmonyMessageBus.sol	0	2	0.0%
contracts/messaging/MessageBus.sol	0	5	0.0%
contracts/messaging/MessageBusReceiver.sol	0	17	0.0%
contracts/messaging/MessageBusSender.sol	0	21	0.0%

Totals
Change from base Build 2386183733:	-0.5%
Covered Lines:	664
Relevant Lines:	2334

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💛 - Coveralls

[ChiTimesChi]

Closed in favor of #162