[Blossom] Resolve interaction between ZIP 308 (Sprout to Sapling Migration) and shorter block times

[daira]

ZIP 308 specifies that migration is done every 500 blocks; decide whether this needs to change as a result of ZIP 208 (Shorter Block Target Spacing).

[daira]

One option is to simply leave it as-is, and accept that this will result in migrations occurring more frequently after Blossom activation.

The 3-hour window introduced in #3995 also needs to be reconsidered, based on whether we change the two-hour limit on block timestamp postdating.

[daira]

The 3-hour window need not be changed because, according to the rationale in #3995 (comment), it only needs to be greater than the 2-hour limit on future-dated block timestamps, which did not change. (If we were to change that limit it would only be to decrease it.)

In any case, neither the 500-block interval nor the 3-hour window were changed in the implementation. However, I missed updating ZIPs 208 and 308, and will do that now.

[daira]

ZIPs 208 and 308 have been updated in zcash/zips@20ebcf4