Fix security settings in E1
While testing E1 with security enabled for DRBs, it was found that data traffic was not functional.
The problem was that security settings were modified in the PDCP reestablishment happening when receiving Bearer Context Modification Request.
So some restructuring was done.
The actual fix is in 18176297, the others are for support. Basically we introduce a boolean to check if the security settings are present in Bearer Context Modification Request and we change PDCP security settings only if they are.
(Note that in OAI CU security settings are not sent in Bearer Context Modification Request, so we could also remove all this, but we need to be compatible with other cu-up and/or cu-cp.)