Change ProtobufMessage internal API

We should use 'get', 'set', 'append', 'clear', 'count' and 'fields' methods instead of 'getValue', 'setValue', 'appendValue', 'clearValues', 'countValues' and 'getFields' to avoid naming conflicts (when using protos with "Values" or "Fields" name for example). Method 'clear' renamed to 'reset' as well.

Existing protos must be recompilated. And yes, there could be some backward compatibility issues if someone uses this methods but there is no other way to resolve such conflicts...