[bug] Use median-time-past for input sequence checks rather than block time

[m-kus]

Problem

Currently OutPoint contains block_time field which is used to validate relative locktime — when the TXO can be spent only if enough time have past since the block containing that TXO.

However in Bitcoin core they use MTP (median time past) for that purpose: https://github.com/bitcoin/bitcoin/blob/712a2b5453cdf2568fece94b969d6e0923b6ba16/src/consensus/tx_verify.cpp#L74

MTP is the median of previous timestamps for the chain state preceding the target block.
We have this data, we just need to propagate it down to the place where it's needed.

Fix

• Remove block_time field from outpoint and add new median_time_past field.
• Compute MTP once (currently it is done in validate_timestamp, so it can be a ret value) and then use for every tx input
• Update data generation scripts accordingly and regenerate tests

[fishonamos]

Kindly assign @m-kus