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fix: make bootstrap more robust (and slower 😢) (#1351)

This diff ensures that the boostrap is more robust in the scenario
described by https://github.com/ooni/probe/issues/2544.

Because we're close to a release, I did not want to write a heavy
refactor of the boostrap, and specifically of the iplookup part, but I
documented what is needed at https://github.com/ooni/probe/issues/2551.

This diff has been tested in a problematic network under two distinct
conditions:

1. cold bootstrap (i.e., w/o prior knowledge), where the correct fix is
to give the system resolver intermediate priority (i.e., 0.5);

2. hot bootstrap with very f***** up `sessionresolver.state`, where the
correct fix is to use a larger coarse grained timeout, such that
_eventually_ we try the system resolver.

The fix that prevents marking a resolver when the external context has been
canceled is required in both of these scenarios.

As you can see, I removed a "reliability fix", which actually was more
of an optimization. This removal means that the probe bootstrap is now
slower than it could be, hence my choice of documenting in
https://github.com/ooni/probe/issues/2551 what I'd do had I had more
time to work on this topic.

BTW, I had to create an overall 45 seconds timeout for IP lookups
because we have 7 DNS over HTTPS resolvers plus the system resolver.
This means that, in the worst case where the system resolver has the
least priority, we expect 7*4 = 28 seconds of timeout before eventually
using the system resolver. The rest of the 45s timeout accounts for
operations happening after the DNS lookup has completed.

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