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