[refactor] track pending vault transactions

The code currently makes an assumption that between transactions of the migration of funds between faults, that the transactions are completed, before the next round(s) of migrations.

While this is likely to be true, its prob not a good assumption to make.

This code change keeps track of how many pending transactions there are so that when the code goes to migrate more funds it can know if the previous migration is complete or not.

In addition, just for giggles, we're also now tracking how many inbound/outbound transactions each vault sees.