General Improvements to the CrossChainTransferStore

[bokobza]

This class is big and devoid of logs. We need to

• simplify it
• add unit tests
• add more logs

[quantumagi]

The CrossChainTransferStoreTests class already contains a number of unit tests that test such issues as:

• signature merging
• withdrawal transaction creation
• reorgs
• recovery from chain
• wallet synchronization
• transaction and signature validation

so this is the appropriate place to add any additional tests.

Alternatively additional test classes can leverage the CrossChainTestBase class.

[quantumagi]

I've added some notes on simplification to #279. Let me know your thoughts on this:

• Let go of the idea of broadcasting transactions between members in a loose asynchronous manner.
• Have a single method (it could call other methods) used by the leader that:
  • Builds a transaction
  • Passes the transaction among members via parallel synchronous calls for signing.
  • Broadcasts the transaction if enough signatures were collected.
  • This will require a quorum to be online simultaneously but should not be too big an ask.