Add a constant for the maximum vector allocation (5 Mbyte)

Summary:
This is a partial backport of Core PR18021 [1/4]
Commit https://github.com/bitcoin/bitcoin/pull/18021/commits/37d800bea016d5cba5635db036f53a486614ed30

Test Plan: ninja all check all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8742