BUG/MINOR: resolvers: Wait the resolution execution for a do_resolv action

BUG/MINOR: resolvers: Wait the resolution execution for a do_resolv action

The do_resolv action triggers a resolution and must wait for the
result. Concretely, if no cache entry is available, it creates a resolution
and wakes up the resolvers task. Then it yields. When the action is
recalled, if the resolution is still running, it yields again.

However, if the resolution is not running, it does not check it was
running. Thus, it is possible to ignore the resolution because the action
was recalled before the resolvers task had a chance to be executed. If there
is result, the action must yield.

This patch should fix the issue #1993. It must be backported as far as 2.0.