Avoid downloading large sprout param files

[leto]

NOTE: This is a WIP and is not ready to be merged.

This patch allows a HUSH node to avoid downloading ~900MB of Sprout param files and fully sync without any problems, because the new HUSH mainnet has no Sprout transactions.

For Pirate, this gets us most of the way there, but more work needs to be done. My trick with Hush was to never initialize the data structures which store old Sprout params. With Pirate, some code might need to be modified to prevent dereferencing a null pointer.

Currently Pirate can sync 32 blocks and then coredumps in block 33 when trying to parse the first JoinSplit on the chain, inside CheckTransaction() :

https://gist.github.com/leto/32344417f28688f20810cecd3f0ec655