RangeFinder: allow two (or more) serial lidar to be used at the same time

[rmackay9]

We should support using two serial range finders at the same time.

The user should be able to define which two serial ports to use by setting multiple SERIALx_PROTOCOL parameters to "9" meaning "Lidar". i.e. SERIAL1_PROTOCOL = 9, SERIAL3_PROTOCOL = 9 to mean the first serial range finder should use serial port1, the 2nd should use serial port 3.

The reason that we can't currently is that our serial Lidar drivers (like the LightwareSerial driver) decide which serial port to use by asking the serial manager but it always asks for the first serial port configured for Lidar. Instead the serial lidar driver needs to know how many other serial lidar drivers have already been created and then ask for the next free serial port.

This is where the Lightware Serial driver is constructed: https://github.com/ArduPilot/ardupilot/blob/master/libraries/AP_RangeFinder/RangeFinder.cpp#L676
Serial manager: https://github.com/ArduPilot/ardupilot/tree/master/libraries/AP_SerialManager

[rmackay9]

Tested today with some patches from @OXINARF. looking good!