Cache on-demand proxies

[briancavalier]

Wire doesn't create a proxy for a component it didn't create. So, sometimes, it has to create a proxy on-demand, when a plugin asks for it. Prime example: DOM nodes can be pulled into the current context via reference resolver. Since wire didn't create the DOM node, it doesn't proxy it by default. If another component uses the on facet (via wire/on) to connect to an event on the DOM node, wire/on asks for the node's proxy, and wire has to create it right then. Currently, we don't retain the association between the node and the just-created proxy. Perhaps we should.

[scothis]

If for some reason the creation/destruction of the DOM node isn't in sync with the wire spec, this could create a memory leak.