Compute script flags based on the chain tip rather than the block we are connecting.

Summary: This fixes T288 and ensures that flags are consistent when accepting new block and when accepting txns in the mempool without having to build extra piping.

Test Plan:

make check

Do IBD on mainnet and testnet

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Maniphest Tasks: T288

Differential Revision: https://reviews.bitcoinabc.org/D1271