Restarting flashblock rollup-boost causes sequencer to stall

[cody-wang-cb]

Running the flashblock branch #98. With --flashblock flag on and websocket enabled.
Noticed that if rollup-boost is restarted, it would cause op-node to go to paused state and stopped sequencing.

To reproduce

• Start everything (rollup-boost with --flashblock flag and websocket on, op-stack, rbuilder)
• Let it run for a bit
• Shut down rollup-boost, restart it
• Rbuilder will stop building flashblocks after a few seconds, op-node stops sequencing with "sequencer paused" message

Interestingly, if the logic of using flashblock payload is commented out and have it to make a get_payload call to the builder, the sequencer doesn't stall anymore.

[cody-wang-cb]

Using this to disable the use of flashblock payload for now
#149