[avalanche] Add a facility to select invs to poll

Summary:
This extends the AvalancheProcessor with facility to decide what to poll.

Depends on D2043

Test Plan: Added unit tests.

Reviewers: #bitcoin_abc, schancel, Fabien

Reviewed By: #bitcoin_abc, schancel, Fabien

Subscribers: Fabien, jasonbcox, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D2044