Handle deletes on the backlog table

[r-marques]

We need to create a changefeed that listens to deletes in the backlog table. Deletes in the backlog should only be allowed if the transactions were included in a block in the bigchain table.

The voter code right now deletes the transactions as it processes them which means that there will a some time gap between the time a transaction is deleted and the time the block makes it to the bigchain. I can think of two solutions for this:

1. When a transaction is deleted cache it and check the bigchain table for the transaction after a timeout period (e.g. 30 seconds). If after the timeout period the transaction is not in the bigchain re-insert the transaction in the backlog.
2. The node creating the block first waits for the block to be written to the bigchain and them deletes the transactions from the backlog.

The second solution seems more robust since it does not rely on a timeout period but it requires changes to the voter process

[rhsimplex]

Related to #568 and #198, closing to consolidate discussion (see #568)