Use switch(string) for operation dispatch in Java and C#

[bernardnormier]

Ice currently performs a search for the operation name like in C++, and then a switch on the position of this string. It could and should use instead a plain switch on the operation name string, and allow the compiler to optimize the switch.

See for example: https://github.com/zeroc-ice/ice/blob/3.7/java/src/Ice/src/main/java/com/zeroc/Ice/Object.java#L154