Track Transaction across message passing

[binaryseed]

Right now a "Transaction" is tracked across spawned processes, so work done in the child process is included in the Transaction.

It would be useful if we could safely track work done during message passing to the Transaction. There are risks however:

• The process would need to be "untracked" as soon as the message is handled
• All message passing is async, so the Transaction might be done already by the time the message is handled

Using seq_trace in addition to the current mechanism might enable this...

From #249