[avalanche] Add handling of ava_poll command in the network layer

Summary:
This is not quite suffiscient by itself as it has no DoS protection, but can answer queries about blocks.

In order to avoid any problem, we make sure that it is gated behind a flag that is turned off by default.

Test Plan:
Added an integration test that poll the node and check answers.
ran bitcoind -help to verify avalanche option do not show up.

Reviewers: #bitcoin_abc, jasonbcox, Fabien

Reviewed By: #bitcoin_abc, jasonbcox, Fabien

Subscribers: Fabien, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5528