Remove protobufs

This has two fixes:
  * Replace ColumnarMetadata protobuf used in RCFile support by a dynamic protobuf so that binaries work with both protobuf 2.4.x and 2.5.x. This fix is similar to #373 to fix #358
  * Remove CountedMap protobuf. This was used to support 'String -> Long' maps in Pig in a protobuf. I don't think this feature was ever used.. especially after EB added support for Thrift (in early 2011), which has maps. This would be a backward incompatible change, but I don't know any known script affected by this.