Make it possible to pass around `&mut <path related state>`

Problem

Currently, we call .get(&path_id) and .get_mut(&path_id) again and again.
We do so with Connection::paths, Connection::rem_cids, Connection::local_cid_state, Connection::path_stats, and PacketSpace::number_spaces.

We don't re-use the results from .get or .get_mut, because these end up borrowing &mut Connection, during their lifetime we can't call functions that operate on the connection.

Proposed solution

Ideally, we separate out the all the non-path-specific state in Connection from the path-specific state, so that these borrows don't get into each other's way.

Notes

In the past, when we've tried to make this refactor, we found some parts that make this hard, e.g. the pto_max_path that needs to traverse all paths, thus we can't hold a borrow of a path at the same time.
Another issue is when we need to operate on two paths at the same time. That can be required e.g. when we want to send PATH_ABANDONs. We both need access to the path that we're currently abandoning, as well as another open path that we actually send the PATH_ABANDON on. There exists HashMap::get_disjoint_mut, but there's no equivalent for BTreeMap, unfortunately.

Start by tracing the .get and .get_mut call sites involving Connection::paths, rem_cids, local_cid_state, path_stats, and PacketSpace::number_spaces. Read the handling of pto_max_path and PATH_ABANDON operations to understand the cross-path borrowing constraints. Done means path-specific and connection-wide state can be borrowed independently without breaking multi-path operations.