prevent call of success/failure handlers if timeout Fixes #2529997

Remove 300ms wait() limit. Add no-cache headers.

Add allowCache config to reuse the same proxy fn Useful for at least YQL to get the benefit of caching rather than each call to send() generating a unique outbound url.

Simplify url callback parsing, Extract to submod. Split into jsonp-base and jsonp-url. Base does simple, single level name resolution for callback-in-url parsing (callback=foo, or callback=Y.foo). Nested parsing moved to jsonp-url (callback=foo.bar["baz"]).