v0.4.12: add --sync-from-height command-line option

Convenience follow-on to v0.4.11

Releases v04.11 and v0.4.12 both fix the same critical bug (see #392)! It's imperative to upgrade to either of these releases before NU5 activates on mainnet!

If you have not deployed v0.4.11 yet, we recommend upgrading to this release instead. Release v0.4.11 requires you to completely rebuild the lightwalletd cache (see its release notes), which causes several hours of downtime. This release (v0.4.12) makes it possible to correct the cache without having to rebuild it from scratch. This is much faster and easier.

This release adds a new lightwalletd command-line argument option, --sync-from-height <height>. This option causes lightwalletd to drop all blocks at this height and above from the cache at startup, so that lightwalletd will begin syncing from the given height.

This process must be followed for testnet, and also on mainnet only if NU5 has already activated there (expected May 31, 2022, at approximately 1600 UTC).

1. Stop the running lightwalletd (it's not necessary to stop the local zcashd)
2. Upgrade lightwalletd to this release, v0.4.12
3. Restart lightwalletd with the new option, please specify these heights:

• Testnet: Start lightwalletd with the option --sync-from-height 1842420
• Mainnet: Start lightwalletd with the option --sync-from-height 1687104

4. Let lightwalletd finish syncing (it should take only a few minutes)
5. You may leave lightwalletd running, but when it is restarted, don't specify --sync-from-height again, or it will do unnecessary work.

What's Changed

• add --sync-from-height command-line option (#380)