Refactoring the XDM relayer

[NingLin-P]

This PR is a refactoring of the XDM relayer, the change set is quite large so I didn't organize the change commit by commit but the logic after refactoring should be clear and easy to review.

The relayer works as follows:

1. For every imported consensus block
   • Skip if it is in major sync
   • Skip if the block is not the best block
2. Get the finalized_block as imported_block - confirmation_depth_k
   • The finalized_block won't be processed directly but instead the relayer will process finalized_block - 1 and finalized_block is used to generate the MMR proof of finalized_block - 1
3. Fetch and filter XDM in finalized_block - 1
   • The relayer will check against the current best block state to see if the XDM in finalized_block - 1 is already relayed
4. Construct XDM proof which consists of:
   • The MMR proof of finalized_block - 1
   • The message proof
   • If the src chain is a domain there is also a storage proof of the LastConfirmedDomainBlockReceipt
5. Gossip/submit the XDM to the network

A few notable logical changes:

• The relayer of the consensus chain and the domain is merged into one worker
• The relayer when encountering an error will not shut down the whole node but instead just print an error log, since it is not fatal
• The is_state_available check is removed so the relayer just prints an error log if the state at finalized_block - 1 and LastConfirmedDomainBlock is not available

Code contributor checklist:

• [x] I have read, understood and followed contributing guide