Add connection_limits behavior to FuelBehavior

[MitchTurner]

Before upgrading libp2p we managed our connection limits through the SwarmBuilder. That functionality has since been removed from the SwarmBuilder and moved to a dedicated connection_limit behavior:
https://github.com/libp2p/rust-libp2p/blob/master/misc/connection-limits/src/lib.rs

We need to add tests that verify desired connection limiting and then add the behavior to fulfill our requirements :)