Enabled multi-threading for gNB L1 tx
PHY PDUs like PDCCH PDU, PDSCH PDU, SSB PDU and CSI PDU have been removed from PHY_VARS_gNB structure. Instead, they are made as a part of thread message for L1 Tx processing.
Two such thread messages are used so that two independent L1 Tx processing threads can be executed at once.
Monolithic mode works fine with rfsimulator. nfapi mode changes are integrated by not tested. Let the CI test it.