[ConnectBlock] Use an index to refers into blockundo.vtxundo rather than pusing as we go

Summary: This makes the code more solid as there is no risk of reallocation mid way.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5299