This adds request/response event pairs to enable the controller to dispatch
"actions" (pre-implemented Zeek script actions) on subsets of Zeek cluster nodes
and collect the results. Using generic events to carry multiple such "run X on
the nodes" scenarios simplifies adding these in the future.
This provides Broker-level plumbing that allows agents to reach out to their
managed Zeek nodes and collect responses.
As a first event, it establishes Management::Node::API::notify_agent_hello,
to notify the agent when the cluster node is ready to communicate.
Also a bit of comment rewording to replace use of "data cluster" with simply
"cluster", to avoid ambiguity with data nodes in SumStats, and expansion of
test-all-policy.zeek and related/dependent tests, since we're introducing new
scripts.
