refactor: make active_chain_tip a reference

Summary:
This PR fixes a TODO introduced in core#21055.

Makes active_chain_tip argument in ContextualCheckTransactionForCurrentBlock function a reference instead of a pointer.

This is a backport of core#25677

See D144 for the difference in the function name

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16074