Process transaction in reverse order in DisconnectedBlockTransactions::addForBlock

Summary: It odesn't change anything for CTOR blocks, but makes TTOR block slightly faster to process, so that should help IBD.

Test Plan: Do IBD on testnet.

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2105