this rather large patch changes the way cells are handled in the SS.
It moves RLC and PDCP entities to the cell map of the SS, such that each cell
has its own entities. This allows to support HO operation, for example,
in which two cells need to send SRB messages.
It also extends most of the syssim interface to include the cell name
in all commands so they can be applied on the specified cell only.
unfortunately, the rapidjson version shipped with Ubuntu 16.04
doesn't support the GetArray() API so we need to use
normal iterator to loop over the array