BCH May 2023 Upgrade: Allow for txn sizes as small as 65 bytes

Summary

Please see discussion here: https://bitcoincashresearch.org/t/chip-2021-01-allow-transactions-to-be-smaller-in-size

This MR reduces the minimum txn size for Upgrade9 (May 2023 network upgrade) to 65 bytes from the existing 100 byte minimum size that we have had since Magnetic Anomaly activated in Nov 2018.

The original motivation for imposing a minimum size at all was in order to avoid a merkle tree exploit that is possible for txns of precisely 64 bytes in size. However, the size of 100 bytes was chosen arbitrarily and it has been known to at least break some mining software that comes from BTC and is unaware that they need to pad their coinbase txns (often by default they get a 98 byte coinbase).

As such, it would be more useful for everybody if the min. txn size was reduced to 65 bytes. Note that another approach would have been to simply forbid 64 byte sizes and allow for even smaller txn sizes. The smallest possible trivial txn that pays out to an empty script is 60 bytes. However such small txns are not very useful, and it was felt the author of this MR and others that it is easier and more natural to enforce a minimum size of 65, rather than have a single forbidden size of 64. (It also is likely to fit into the existing logic of extant software which was just enforcing a minimum of 100 to simply reduce this constant down to 65 -- in the BCHN codebase it was indeed better to do it this way).

The functional test that was added carefully tests the the 100-byte rule pre-upgrade9 and then moves the blockchain forward to activate upgrade9, at which time it tests that the minimum size has indeed been reduced to 65.

Test Plan

• ninja all check-all