[avalanche] Request for more avalanche nodes when needed

Summary:
This diff spins a new network scheduler task that checks every 5 to 10 minutes whether it is required to request more avalanche nodes, and takes action accordingly.

Since it's running in the scheduler thread, the Peer::m_addr_token_bucket attribute needs to be guarded by a lock. Due to the way it is used, an atomic is not the best option here so a Mutex is used.

Ref T1696.

Depends on D11020, D11033 and D11034.

Test Plan:
With Clang and Debug enabled:

ninja all check-all

Reviewers: #bitcoin_abc, tyler-smith

Reviewed By: #bitcoin_abc, tyler-smith

Subscribers: tyler-smith

Maniphest Tasks: T1696

Differential Revision: https://reviews.bitcoinabc.org/D11035