Support for source seal anchors and discovery for seals that are anchored in Merkle Trees and bound to the KEL via a Merkle root seal.

[SmithSamuelM]

Feature request description/rationale

Currently source seal couples can be used to percolate the less expensive validation of an event that is bound or anchored in some KEL. The two types of events that use this type of anchoring are delegated key events and transaction event log related events like revocation registry events.

When the anchor (binding seal) is placed in the seal list (anchor list) in the a field, a validator can discover it by walking the KEL either forward or backward and comparing every seal in every seal list against the seal (event said and event sn) of the anchored event.

When the validator is given a source seal couple, the couple points to the anchoring event's SAID and SN. This means that instead of walking the KEL to find the anchor, the validator merely needs to compare against the anchors in one event. This is a performance optimization enabled by percolating the source seal couplethat points to the anchoring event by attaching it to with the event being anchored.

When the event is anchored in a merkle treee which in tern is anchored with its merkel root discovery by walking the KEL no longer works. The validator would have have to have all the full Merkle tree or be able to query all the anchored Merkle trees.

So in the future, we may want to impose a discovery requirement that enables validators to query some watcher designated by the Sealer (anchorer) which will have the source seal attached to the anchored event with an API for querying the associated Merkle tree.

Related to this is the current problem of promiscuous witness services that are also acting as watchers for all other kels besides the KEL for which they are the witness. This fails when someone queries the witness for the source seal of a kel for which the witness is a witness becasue the witness does not need the anchoring seal to validate and accept into its kel the anchored event so it discards the source seal should it be attached.

The fix is for the validator to walk the kel of the Sealer and find the anchor or to get a source seal from a valid watcher of the anchored kel or have it percolated to them.