bugfix: change memory management of interface between PHY and MAC scheduler

This commit fixes a bug caused by global variables in the gNB_MAC_INST structure used to transfer MAC scheduler's results to the PHY layer.

When using several threads in the PHY layer, it may happen that the MAC scheduler is called a second time by the second PHY thread for the next slot before the content of the global variables in the gNB_MAC_INST structure used to store the result of the MAC scheduler for the current slot have been consumed by the first thread, leading to unpredictable results (all of them certainly wrong).

The solution is to introduce a small memory management module for the interface between the PHY threads and the MAC scheduler. The MAC scheduler now fills the data structures passed to it, and not a global variable anymore. The PHY layer manages the structures and ensures that no data is overwritten before being fully consumed by the various threads of the PHY layer.

Some care has to be taken because several PHY threads will access the same structures at the same time for a given slot (in read mode, so it's okay). A mechanism of reference counting has thus been introduced.

Only one CC is supported. This restriction should not be too hard to remove if/when needed. (The code before this commit does not seem to work for more than one CC anyway.)