- 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

protobuf-c does not compile anymore.

Let's handle this a bit better.

We now install protobuf and protobuf-c only for the
flexran agent. That is, if you want to use the flexran
agent, you need to install protobuf/protobuf-c and
you do it this way:

  ./build_oai -I -a

(you add -a)

Other targets don't need protobuf nor protobuf-c, so
it's not installed by the -I command of build_oai,
unless you pass -a with -I.

Also, we now use protobuf 3.3.0, not 2.6.1. The code
has been adapted, a quick test seems to indicate that
the system works, but it has not been intensively tested.

Merge branch 'feature-68-enb-agent' of https://gitlab.eurecom.fr/oai/openairinterface5g into feature-68-enb-agent

Summary of changes:
- UE: new thread idx
- UE: Pdcch optim
- UE: slot0 slot1 parallelization
- bugfixes for RLC AM (see gitlab issue 250)
- fix bug "unknown UE_id for rnti"

Note: due to the new UE threading architecture,
oaisim and the phy simulators may not work properly
anymore. Adaptation in the code has been done, automatic
tests seem to pass, but it may not be enough.

See merge request !215

With the new threading architecture of the UE dlsim
(and others) does not work properly anymore.

When looking at the scope, you see a difference
in PDSCH LLR display. The end is always 0 where
in the current develop branch (tag 2017.w25) it's not.

This commit attempts to fix it.

We still don't have the same behavior as in 2017.w25.
I disabled channel simulation (so that UE RX = eNB TX)
and I have one error where in 2017.w25 I have zero.
For example, here comes the output of a run of "./dlsim":

**********************SNR = 0.000000 dB (tx_lev 51.000000)**************************
Errors (1(0)/1000 0/1 0/0 0/0), Pe = (1.000000e-03,0.000000e+00,-nan,-nan), dci_errors 0/1001, Pe = 0.000000e+00 => effective rate 99.900100, normalized delay 0.001472 (1.001000)

And in 2017.w25 we have (with the same hack to disable
channel simulation):

**********************SNR = 0.000000 dB (tx_lev 51.000000)**************************
Errors (0(0)/1000 0/0 0/0 0/0), Pe = (0.000000e+00,-nan,-nan,-nan), dci_errors 0/1000, Pe = 0.000000e+00 => effective rate 100.000000, normalized delay 0.001471 (1.000000)

There may be a problem somewhere. Or there was one before and we should
have had one error and the new UE architecture fixed things and now
it's as it has to be. Hard to say at this point...

When looking at the scope we quickly see some zeros for the PDSCH
LLR, at the begining this time, not at the end. This is just when
the GUI appears and then all is fine, so this seems to be for the
first frame only. In 2017.w25 this does not happen.