Try to merge feature-68-enb-agent into develop
develop should be fast-forwarded later to this branch to complete the merge
This code does not compile yet. Among other things:
- the scheduler structure needs to be reworked for FlexRAN
- the config is structured differently. The files enb_config.{c,h} from develop
  have been copied in here, the files with merge annotations are in
  enb_config.{c,h}.before_merge
- the restart needs to be restructured, since the PHY vars data structure lives
  in RC now

- lock with mutexes
- don't start ENB_APP task, but Agent directly and wait in case of
  reconfiguration

- {restart,stop}_L1L2() in lte-softmodem.c
- add function start_phy_rrc() in enb_app.{c,h}, accessible from outside
- will be used to restart PHY and RRC when lte-softmodem is restarted

use only one FlexRAN wait function

- when exiting task, the task is set as "not configured"
- this permits to start a task again

*Fix the issue with the PDCP UE ID/RNTI management

Summary of changes:
- bug fixes: random access, crash on physicalConfigDedicated)
- some work on the UE to have it functional again
- IQ record and playback (work from Nokia)
  see documentation at: https://gitlab.eurecom.fr/oai/openairinterface5g/merge_requests/244

Conflicts:
	targets/DOCS/oai_L1_L2_procedures.pdf

Merge branch 'fix_ue' of https://gitlab.eurecom.fr/florian.kaltenberger/openairinterface5g into fix_ue