Prior to this this commit, the structure NR_UE_info_t is 4881632 bytes.
On some machines, e.g., one of the CI systems, this can lead to realtime
problems when doing the initial calloc() of the data structure.

Commit e586efb2 ("Enable 32 HARQ
Processes in DL, UL") introduced the real-time problems by increasing
the number of HARQ processes.

The reason NR_UE_info_t is that big is that it contains buffers for DL
HARQ data that might need to be retransmitted (the L1 is stateless, it
cannot store this for the L2). To reduce the size, dynamically allocate
the DL HARQ buffers when they are needed. This also reduces the size of
NR_UE_info_t to 15840 bytes.

Encapsulate transportBlock in byte_array and accessory functions, and
allocate in big chunks of multiple of ~32kB.
Reported-By: Cedric Roux <cedric.roux@eurecom.fr>
Fixes: e586efb2
Closes: #875