In reality, we are not going to use the sessionresolver when we're
using a proxy (I just tested). But, it nonetheless feels a lot more
robust to write a correct sessionresolver that handles the proxy
in the most correct way. That is, the sessionresolver will now skip
all the entries that cannot use a socks5 proxy (including among them
also the system resolver). What's more, it will construct a child
resolver that propagates the proxy.
We have confidence that this holds true because we have added a test
ensuring that we are really using the configured proxy.
See https://github.com/ooni/probe/issues/1381
f0110fe85a