Bump DEFAULT_MAX_PEER_CONNECTIONS to 4096

Summary: With avalanche, we'll have to keep in touch with more nodes.

Test Plan:

ninja all check-all

Kept a node running for a while and check that resource consumption is not out of wack.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8605