Restructure directory and add SCG configuration Protobuf

Changed scg-CellGroupConfig parameter type from Bytes to RRCReconfiguration
to describe SCG configuration as a Protobuf structure. The change affects to
following X2 messages:
* SGNB ADDITION REQUEST ACKNOWLEDGE
* SGNB MODIFICATION REQUIRED
* SGNB MODIFICATION REQUEST ACKNOWLEDGE
* SGNB CHANGE REQUIRED

Moved also all '.proto' files from streaming-protobufs root directory to proto
subdirectory.

Added also Golang source code generated from Protobuf '.proto' files, generated
files are located under protogen/gopb directory.

Signed-off-by: Timo Tietavainen <timo.tietavainen@nokia.com>
Change-Id: Ia5478a0fca83794fff847a0c43a3f8f45ab90b6a