Before this commit, the DLSCH scheduler would construct the MAC PDU by
reading RLC data into a memory on the stack, and then construct the PDU
with CEs first. There are at least two problems:
- we need to keep track of the exact number of bytes of CEs (cumbersome)
  to calculate the number of MAC SDUs to include
- we needlessly copy data around.

This commit does the following instead:
- write all CEs first (no need of keeping track of this in DLSCH and a
  separate function)
- then read MAC SDUs directly into nFAPI as much as possible or
  necessary, without recopying

According to SCF222, a single PDCCH allocation groups DCIs that are
within the same BWP and CORESET. Therefore, if we want to allocate
multiple DCIs, we need to decouple PDCCH allocation and DCI (previously
jointly done in nr_configure_pdcch()), especially to be forward
compatible.

***Note that as of this commit, we would still allocate different PDCCH
PDUs for multiple UEs (which we do not support yet, anyway)***

nr_configure_pdcch(): simply take out DCI allocation.

nr_generate_Msg2(): separately allocate dci_pdu in common RA SS, and
rename DCI payload variable. Also, reorganize the function so that it is
first checked for CCE allocation and messages nFAPI messages are
allocated afterwards.

nr_schedule_ue_spec(): separately allocate dci_pdu in UE-specific SS.
Rename DCI payload variable.

nr_schedule_ulsch(): separately allocate dci_pdu in UE-specific SS.
Rename DCI payload variable.

SCF222 says that dci_pdu in PDCCH PDU
(nfapi_nr_dl_tti_pdcch_pdu_rel15_t) should be an array, and not its
members.

Initially, filling the PDCCH and PDSCH nFAPI messages was split into a
separate function (in an attempt to keep the code structure similar to
LTE). However, this proved as not helpful: the nr_fill_nfapi_dl_pdu()
just filled the messages, with a parameter list almost size as long as
the actual messages (because most parameters are kind of independent).
This made no sense, so we put it back.

Also, from an understanding POV, they just fill a message as specified
in SCF222, so it should not be a problem.