Add event when a ScriptBuf is found at a block

[rustaceanrob]

When changes must be applied in chain order, users may want to know what blocks they will receive at a later time. Emitting this event allows users to either fetch the header or wait for a block at a specific height.

[rustaceanrob]

This would be an O(log(N)) read for a simple log message. Not worth the overhead.