engine_executePayload QoS when sync triggered

[djrtwo]

When running engin_executePayload, CL might trigger EL to sync. This might be a huge sync that takes a long time or it could be a short-range sync of filling in a few blocks.

In #84, we discussed potentially adding some notion of how long EL can take to retrieve data from the network before it decides it is SYNCING and can't respond with VALID/INVALID.

This brings in additional complexity and only optimizes these (likely) rare cases when EL and CL have a block or two inconsistency issue.

I'm leaning toward not adding the note at all, but am dropping this for discussion before a final decision is made.

Some relevant discussion points here -- #84 (comment)

[djrtwo]

closing this as deemed unnecessary