[avalanche] Improve readability of the multi index container definitions

Summary:
This makes use of a namespace alias to make the code slightly more readable.

There is no change in behavior.

Ref T1634.

Test Plan:

ninja all check-avalanche

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Maniphest Tasks: T1634

Differential Revision: https://reviews.bitcoinabc.org/D9738