resolved: when restarting a transaction make sure to not touch it anymore (#3553)

dns_transaction_maybe_restart() is supposed to return 1 if the the transaction
has been restarted and 0 otherwise. dns_transaction_process_dnssec() relies on
this behaviour. Before this change in case of restart we'd call
dns_transaction_go() when restarting the lookup, returning its return value
unmodified. This is wrong however, as that function returns 1 if the
transaction is pending, and 0 if it completed immediately, which is a very
different set of return values. Fix this, by always returning 1 on redirection.

The wrong return value resulted in all kinds of bad memory accesses as we might
continue processing a transaction that was redirected and completed immediately
(and thus freed).

This patch also adds comments to the two functions to clarify the return values
for the future.

Most likely fixes: #2942 #3475 #3484

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