[RPC] Fix Invalidateblock

[panleone]

Invalidateblock has not been working since sapling came out. This happens because the current maximum witness cache has a depth of only 100 blocks. With this PR I want to fix this issue by manually building the witness cache if the block has a depth bigger than the cache size.
Since this operation can become pretty lengthy, the RPC will just return error if your oldest sapling note is older than 1 month (43200 blocks behind the chain tip).

Note: After using invalidateblock the user might need to do
-> debug ->wallet repair -> recover transactions in order to recover the correct balance. This is not a limit of this PR and you can test that it already happens with the current implementation of invalidateblock.
Anyways "recover transactions" also cleans the wallet GUI from "conflicted" txs that you get after invalidating a block so it would be useful regardless.

Overall invalidating + eventually using recover transactions is faster than the current rescanning the blockchain from 0
Solves #2605

[Duddino]

I think it would be better if you put the max sapling note age in the RPC, because for wallets with sapling notes older than 1 month, the alternative is to rescan everything from scratch, which is going to take longer anyways

[panleone]

thanks for the review Duddino! I rebased with your suggestion...

I think it would be better if you put the max sapling note age in the RPC, because for wallets with sapling notes older than 1 month, the alternative is to rescan everything from scratch, which is going to take longer anyways

I don't totally agree on this, from PR #2820 users will also be able to rollback to the last checkpoint which is quite fast. So I'd keep the limit of 1 month or in case extend it to 2 months

[panleone]

Update: found an optimization that makes the rpc call much faster so I decided to remove the upper bound limit of 1 month.

On my regtest it took 15 seconds to invalidate a block and in the wallet I had 15 sapling notes each with a depth of around 70k blocks.

[panleone]

rebased on master

[Liquid369]

tACK be49720

[Fuzzbawls]

ACK be49720