Notable changes

Switch to ed25519-zebra for consensus signature checks

This removes the zcashd dependency upon libsodium for ed25519 signature checks and instead uses the Rust implementation in ed25519-zebra, which has been active for signature verification since the Canopy upgrade. For more information on the conditions that led to this change see https://hdevalence.ca/blog/2020-10-04-its-25519am.

Update default fees according to ZIP-313

Reduce default fees to 0.00001 ZEC as specified in ZIP-313 and ensure that transactions paying at least the new minimum fee meet the transaction relay threshold irrespective of transaction size.

Improve getblocktemplate rpc performance when using shielded coinbase

This change precomputes future block templates to permit miners to begin working atop newly arrived blocks as quickly as possible, rather than waiting for a new template to be generated after a block has arrived. It also reduces the initial wait time for incorporating new mempool transactions into a block from 1 minute to 10 seconds; the previous value was inherited from the upstream bitcoin codebase but is inappropriate for our block timing.

Migrate from rpc-tests.sh to rpc-tests.py

This unifies and simplifies the RPC testing framework, as has been done in the upstream Bitcoin codebase.