pan: fix pathPool query path procedure + close underlay UDPConn if QUIC session was not established successfully

[JordiSubira]

There was a bug in the procedure for querying paths, instead of querying paths provided the pathRefreshMinInterval had passed, it did it the other way round. I made use of this PR to refactor the internal procedure. In addition, I also added closing the underlay UDPConn if QUIC session was not established successfully.

While troubleshooting the library I also realized that the library is a bit messy and it would be great to spend some effort in refactoring and simplifying it as possible. We kept postponing this, partially, because we discussed about merging scionproto/snet and the pan library.

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This change is 

[FR4NK-W]

Thank you. It is now also easier to follow what happens 👍

But I believe there is a change in behavior (in addition to the fixing of the refresh after/before expiry):
refresh(dstIA, paths) l.128 is now called on the subscribers regardless of the paths having been updated or not, and regardless of the timer.

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @juagargi and @marcfrei)

[JordiSubira]

Thanks François, you're right, if the paths aren't fetched, it isn't necessary to call refresh on every subscriber (although it is an idempotent operation). I return a boolean flag, to indicate whether the paths are fresh, i.e., recently fetched. It isn't the most elegant solution, but in any case, I am not a big fan of how the library looks like right now and some heavy lifting is anyway necessary.

Reviewable status: 1 of 3 files reviewed, all discussions resolved / -1 of 2 LGTMs obtained / 0 of 2 approvals obtained

[FR4NK-W]

Reviewed 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @juagargi and @marcfrei)