Activate P2SH based on MTP.

Summary:
Numerous checks in GetBlockScriptFlags are based on pindex, rather than pindex->prev. This is a problem because only pindex->prev is available when accepting things into the mempool and generally makes the set of flags undecidable from the tip, but require the block being connected.

This changes the time when P2SH activates on testnet, but this isn't that big of a deal because we don't expect a reorg up to 2012 to happen anyway.

Test Plan: Do IBD on etstnet and mainnet.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1262