add an option to only trigger a predicate after n block confirmations

[MicaiahReid]

It can be confusing for a user to

• get a trigger saying a block matched their predicate
• get that block rolled back
• get another block applied that matches their predicate

Some users may want to keep a live database of the chainstate as it pertains to their predicate, so the current implementation is great.

Other users may want to know "when is my predicate actually confirmed". They don't want to keep any chain state, they just want to be notified of some condition, and they only want it when that condition is confirmed.