This code has not been tested with a COTS UE. We need SRB1/2 (so we need SA)
to test.

The activation of security and integrity for SRB1 will need more code
(new functions) because it happens after some time, not at creation time
of the PDCP entity and there is no way to do this with current code.

Implement data transfer (38.331 5.2):
    - transmit operation (in nr_pdcp_entity_recv_sdu)
    - receive operation (in nr_pdcp_entity_recv_pdu)
    - t-Reordering expiration

To keep track of time the function nr_pdcp_wakeup_timer_thread has been
added and is called by the gNB's MAC scheduler via nr_pdcp_tick.

(The UE has not been modified.)

A thread has been introduced to set time of all the existing PDCP
entities in the system. This probably does not scale well with many
PDCP entities (many UEs connected to the gNB). To be changed at some
point when needed. Hopefully, with a few UEs this mechanism won't be
too problematic (ie. won't cause realtime problems).

The problem with time in PDCP is that SDUs are kept if some are
missing, to do reordering. A timer is started and when it expires
we have to transmit the SDUs even if some are still missing. We
want to do that as soon as possible. So we have to check timers
often (every millisecond).

In RLC the time is set when there is some activity (SDU or PDU
received). In PDCP we cannot wait for this kind of activity. We
would keep packets longer than necessary, which may disturb
upper layers (think: TCP). That is why we introduced the thread,
as a simple mechanism to keep packets no longer than necessary.

The locking done by this thread may create realtime issues.

The whole logic of time keeping can be changed later if it turns
out that it is not efficient.

The functions dealing with DRB AM have been removed. All the types
of PDCP entities (DRB AM, DRB UM, SRB) use the same mechanism for
transmit and receive operations. So the code has been made common
for all those types.

We don't deal with:
    - entity re-establishment, entity release or entity suspend
    - SDU discard (the timer is present but not used)
    - status reporting
    - data recovery
    - data volume calculation
    - header compression / decompression

There is no plan to implement those features in a near future.

Integrity protection is not done yet, and will be implemented soon.

The code is forced to use nea2, no matter what the UE supports.

After 2^18 PDCP packets, it will fail to work (we don't use HFN yet).

These limitations will be fixed in later commits.

The existing security function was not reused, because it does too
much memory allocation and initializes the security context at each
ciphering. So here comes nr_pdcp_security_nea2_cipher(). And also
the ciphering is done inplace. To be changed if necessary.

- Giving C-RNTI as an input to DRB preconfiguration
- Set C-RNTI from UE configuration for higher layer function calls

There was a problem with SN size. It was hardcoded to 12 bits (so max
sn = 4095) but we configure the UE with 18 bits. After a while, iperf
didn't work anymore because of this (what a surprise!).

So let's get the correct value from pdcp_Config.

And while we're at it, let's store also discard_timer and t_reordering
that will be used later by a proper NR PDCP layer (which is currently
work in progress).

This commit contains work done by Eurecom and by Benetel.

-Remaining LTE dependency in RLC add_srb function (not used for NSA setup)
-Rename function for DRB static preconfiguration used in noS1 mode

Initially targeting ENDC setup with commercial UE
- Missing nr_rrc_pdcp and nr_rrc_rlc configuration wrt the established DRB
- Remaining to test with user plane data traffic (Downlink and Uplink)