This adds support for retrieving the value of a global identifier from any
subset of cluster nodes. It relies on the lookup_ID() BiF to retrieve the val,
and to_json() to render the value to an easily parsed string. Ideally we'd send
the val directly, but this hits several roadblocks, including the fact that
Broker won't serialize arbitrary values.
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.
