util.hashring: Support associating arbitrary data with nodes

In this API, a 'node' is always a simple text string. Sometimes the caller may
have a more complex structure representing a node, but the hash ring is really
only concerned with the node's name.

This API change allows :add_nodes() to take a table of `node_name = value`
pairs, as well as the simple array of node names previously accepted.

The 'value' of the selected node is returned as a new second result from
:get_node().

If no value is passed when a node is added, it defaults to `true` (as before,
but this was never previously exposed).

spec/util_hashring_spec.lua

@@ -83,4 +83,11 @@ describe("util.hashring", function ()
 		end
 	end);
 
+	it("should support values associated with nodes", function ()
+		local r = hashring.new(128, sha256);
+		r:add_node("node1", { a = 1 });
+		local node, value = r:get_node("foo");
+		assert.is_equal("node1", node);
+		assert.same({ a = 1 }, value);
+	end);
 end);