Extending the upper value of ref_block_num in tapos beyond 65535 blocks

[aaroncox]

Currently the ref_block_num used in tapos generation has a maximum of 65535 blocks.

https://github.com/EOSIO/eos/blob/58f90b9666dc37cf35eac5dafb6db1c4138024e7/libraries/chain/include/eosio/chain/transaction.hpp#L61

This sets an upper limit on the maximum expiration of a transaction to around 9 hours. It would be beneficial to some off-chain applications (msigs in particular external of eosio.msig) if this maximum timeframe could be extended to multiple days or potentially weeks.

The goal in changes to the system would be that:

• Expiration of a transaction could be set to a long period of time, allowing collection of signatures from multiple parties to satisfy its authority requirements.
• The transaction could be broadcast at any time within that long expiration window (which would just be adjusting the 3600 window in the blockchains configuration).

Just as an example use case, I've thought about how a social account recovery system could be implemented that operated off-chain. Within the current set of rules, the entire account recovery process would need to occur within this designated 9 hour timeframe - which could potentially make the system more difficult to use the user needed to collect signatures from parties in different parts of the world. If a 7 day or 14 day window was possible on the expiration, it would then allow a reasonable amount of time for recovery processes like this.

[heifner]

Context: https://t.me/c/1139062279/260225

We discussed adding a transaction extension to support this protocol feature. We also discussed storing a spare array of ref blocks of say every 10 blocks or every 100 blocks.