Add @SafeVarargs to PromiseAggregator#add and PromiseNotifier#(...)

https://github.com/netty/netty/issues/3147

Motivation:

8fbc513 introduced stray warnings in callsites of
PromiseAggregator#add and PromiseNotifier#(...).

Modifications:

This commit adds the @SafeVarargs annotation to PromiseAggregator#add
and PromiseNotifier#(...). As Netty is built with JDK7, this is a
recognized annotation and should not affect runtime VM versions 1.5 and
1.6.

Result:

Building Netty with JDK7 will no longer produce warnings in the
callsites mentioned above.