Transmit Queue for VarAC Instances that share a resource

Background: A Hermes Lite 2 SDR, with SparkSDR client, can monitor four receivers on four bands simultaneously. However there is only one shared transmitter, and can be only one transmit lowpass filter active at a time.

The problem:

Currently, when running a VarAC Cluster in this configuration, different VarAC instances can suddenly beacon or try to accept a connection, while another VarAC instance is in the middle of transmitting. This causes a conflict! It causes LPFs to suddenly change configuration, and can interrupt the current transmission/QSO.

Solution: 1. Define groups of instances within a VarAC Cluster that share a common resource, e.g. the same transmitter/LPFs/antenna.

Define a conflict resolution mechanism, that ensures that they will never try to use that resource (e.g. transmit) at the same time. For example, a semaphore. Only one instance shall hold the transmit permission at a time.
Define a queue of instances that are waiting to transmit.
Delay transmit of the other instances until the active instance is idle for a period of time - and definitely do NOT transmit while an instance in the cluster is connected to another station!
Once that instance is idle and disconnected, it releases control of the transmitter, and then the next instance instance in the queue can take control of the transmitter safely, and beacon, answer a connection request, etc. And so forth for the next instance, that is waiting in the queue to transmit.
This feature shall be off by default, e.g. to be enabled only for a radio that has multiple receivers and a shared transmitter. For example, the group could be defined and feature enabled as part of the VarAC cluster definitions.
Note that there could still be additional radios in the same cluster. Some may share resources, and others may NOT share any resources.

Example: A VarAC Cluster consisting of 1. A Hermes Lite 2 SDR with Spark SDR software client. It supports up to four receivers, but only has one transmitter that is shared between all four "subradio" instances (radios/receivers/slices).