Consider introducing checkpoints to shorten initial header validation

[tnull]

Currently, we initialize all chain listeners with the genesis hash, which results in a complete validation of the header chain upon first startup.

We should consider a way to cut this short. Unfortunately it's not as easy as polling the best tip and initializing the chain listeners with the most-recent tip as we don't have the chain source available during initialization in the builder (cf. #414 (comment)).

However, one way to improve the initial startup time could be to introduce a hard-coded, more recent, checkpoint with which we initialize all the chain listeners.