[Rollup node manager] encapsulate sequencer functionality

[jonastheis]

I think this is fine for now but I think we can introduce a new type or trait (BlockBuildingTrigger) that can be enabled / disabled such that we don't require the block_time_config to be stored.

Originally posted by @frisitano in #289 (comment)

We might even extend this and also add the sequencer instance into this object so that all sequencer related functionality is in a single object.

[jonastheis]

#289 (comment)

This should also be addressed as part of this issue. It might be appropriate to introduce a component SequencerController that handles all of the logic