Allow multiple calls to SetStatisticsHandler / SetLogHandler / SetErrorHandler

[g7ed6e]

The LogHandler and the StatisticsHandler are protected internal non virtual multicast delegates. This visibility and the fact that current implementation throw when a SetXXXHandler is called multiple times defeats the purpose of multicast delegates and make implementation of a 3rd party package using these delegates fragile. I’m thinking there to the implementation of an instrumentation packages for example.