Fix NPE if no controls available matching configured property filter.
No need for @Resource, this isn't auto-wired.
Add missing property configuration for InstructionHandler list.
Add comments.
Move ModbusConnectionCallback to top-level interface.
Add lock so Modbus connection can safely be used across different
threads. The control.modbus.toggler and control.sma.pcm bundles, for
example, might share the same port.