yet another intermediary merge of x2 into develop

This is not the end of the X2 story. This merge request will not be the last one for the x2 branch.

This merge request introduces one important change in the RRC layer (commit ce84f718) that may impact the overall performances.

Apart from that, some more work has been done on x2.

With this merge request, the x2 handover is functional. What is missing is to deal properly with failures. And there are some bugs that also need to be fixed. Some may be purely due to x2, some may simply have been revealed by the x2 handover (and especially cases where the handover does not run smoothly).