This is a backport of PR10699 ( https://github.com/bitcoin/bitcoin/pull/10699/files ),
but has to be adapted slightly since we don't just do softforking flags.
Pull request description:
This change makes `SCRIPT_VERIFY_UPGRADABLE_NOPS` not apply to `OP_CHECKLOCKTIMEVERIFY` and `OP_CHECKSEQUENCEVERIFY`. This is a no-op as `UPGRADABLE_NOPS` is only set for mempool transactions, and those always have `SCRIPT_VERIFY_CHECKLOCKTIMEVERIFY` and `SCRIPT_VERIFY_CHECKSEQUENCEVERIFY` set as well. The advantage is that setting more flags now always results in a reduction in acceptable scripts (=softfork). This results in a nice and testable property for validation, for which a new test is added. This also means that the introduction of a new definition for a NOP or witness version will likely need the following procedure (example OP_NOP8 here) * Remove OP_NOP8 from being affected by `SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS`. * Add a `SCRIPT_VERIFY_DISCOURAGE_NOP8`, which only applies to `OP_NOP8`. * Add a `SCRIPT_VERIFY_NOP8` which implements the new consensus logic. * Before activation, add `SCRIPT_VERIFY_DISCOURAGE_NOP8` to the mempool flags. * After activation, add `SCRIPT_VERIFY_NOP8` to both the mempool and consensus flags.