Improve masp scan speed

[opsecx]

suggestions to improve masp scan speed (shielded sync):

• if sync fails on fetch (where currently it's highly volatile), have a cache of already fetched data, so it can continue where it left off before halt or
• make sync more resistant to timeouts
• use several cpu threads on scan step after having fetched data. atm the scan part is too "nice" on system resources. this will obviously not improve scan on order/scale (O(n)), but will still cut down time to a lesser factor.

(will be submitting some of these ideas as s-class so don't bother if you see this. if someone else has already suggested these steps lmk and I'll retract)

[brentstone]

Will close this here, but tracking in #2900.