Feature slice+restart on top of develop.w35

This branch includes a couple of features and bugfixes:

• slicing support in the OAI MAC scheduler, to be configured through FlexRAN
• improved start feature, to be initiated by FlexRAN
• fix: make FlexRAN functional on top of develop
• fix: correctly stop all threads at the end of the lte-softmodem
• fix: load_module_shlib() can be called on the same library without creating buffer overflows (needed for restart)
• fix: FlexRAN handles the different IDs (MAC, PDCP, RRC) correctly
• fix: if UDP incoming traffic is too high, the PDCP now blocks this. Before, UE throughput could suffer in this case.

Note: this merge request includes the commit of merge request !379 (merged)