Fixed ERRAI-646: Destroying beans that have observers no longer causes unconditional CDI remote unsubscribe messages

We now first check if other matching local observers are present before unsubscribing from remote events of that type and qualifiers.
Merge branch '2.4'
Upgraded to MVEL 2.1.6.Final
Additional fix for ERRAI-646: Avoid resetting SSE channel every 60 seconds

- For SSE, we now send an empty JSON array as data every 45 seconds to avoid inactivity timeouts
- SSE support was removed from StandardAsyncServlet
Upgraded to MVEL 2.1.7.Final
Fixed ERRAI-277: NPE when proxied class has public method in constructor

Proxies now contain an initialization variable which is set true when
the proxy is closed. If any overridden method is invoked on an unclosed
proxy, it is a NOOP.
Updated ProxyMakerTests to reflect ERRAI-277 changes.
Removed comment to deprecate FROM_UI because it turned out to be useful (see ERRAI-643)
Merge branch '2.4'
Move ProxyUtil to errai-codegen and reuse in ERRAI-277 fix