[avalanche] Move code around in the peer manager

Summary: It's a total mess in there. For some reason, the orphan pool is in between teh slots and teh peer set, and so on. Order don't match int he implementation and header.

Test Plan:

ninja all check

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9750