Merge branch 'ldpc_encoder_decoder' of https://gitlab.eurecom.fr/hongzhi/openairinterface5g into ldpc_encoder_decoder

Conflicts:
	openair1/PHY/CODING/ldpc_encoder.c

moving ldpc_encoder to PHY/CODING
input and output buffers to ldpc_decoding are now allocated outside ldpc_decoding

some minor changes

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.

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