Add statistics variables (with iocShell and EPICS Device Support interface) #193
Description
When debugging production systems
There's a very noticeable complete lack of runtime statistics.
How often does the IOC receive updates on a specific subscription?
Updates with many items?
What's the data rate?
Update loss rate?
...
OPC UA needs statistics
A reasonable set of counters and rates, per item, per subscription, per session, with a command line interface in the iocShell and a Device Support interface to monitor and archive key figures.
A generic Statistics Value class
Something that supports named counters and rates and time measurements (for profiling), using atomics, with a generic device support and iocShell integration, so that it can be used for both UASDK and open62541.