Move monitor-generated HTLC event handling to manager event-getters

This is somewhat awkward, but prevents a slew of duplicate events.
Really this should probably be more explicit, but would be easy to
move that along with a slew of block_connected-event-processing
refactors, see-also GH lightningdevkit#80.

This affects full_stack_target only on accident cause the demo test
didn't continue onwards with another block connection.

Author: TheBlueMatt