Some of these perf counters were useful during initial development, but realistically aren't needed anymore, some are redundant when we can now see the average interval from `work_queue status` and some of them simply aren't worth the cost at higher rates.
* adds a work_queue systemcmd that will bring a tree view of all active work queues and work items
* WorkQueues now track attached WorkItems and will shutdown when the last WorkItem is detached
A board only needs to define:
#define RC_SERIAL_PORT "/dev/ttyS4"
Then it can optionally define one or more of the following:
#define RC_SERIAL_SWAP_RXTX
#define RC_SERIAL_SINGLEWIRE
#define RC_INVERT_INPUT(_invert_true) px4_arch_gpiowrite(GPIO_SBUS_INV, _invert_true)
