rewrite `pigpio` code to `pigpiod_if2`

[rursprung]

we currently start a pigpiod as part of our own process. this has several drawbacks:

• "there can be only one!": we cannot have two packages interacting with the GPIO pins separately (e.g. one the movement & gun controllers)
• there's already a pigpiod running on the default port 8888, so we currently have to manually set ours to 8889
• if our application doesn't terminate properly we have to kill it manually to be able to start it again
• we need to launch our node which uses pigpio with sudo for it to work

instead of using our current approach with the C interface we can use pigpiod_if2. this is an API which connects to an already running deamon (and in our case it's always already running as it's started as a service) and sends the commands there. this allows us to start an arbitrary amount of nodes which talk to the GPIO pins and they don't need sudo rights.

[rursprung]

@joel5399: i've created this as an alternative to what you did in #55 (not the gun part, just the GPIO part). i think this is much simpler & cleaner and would also solve #46. i think rewriting #55 with this pattern should be fairly easy (i could also do that if you'd like).

note that at this moment it's completely untested, we'll have to do that once the robot is recharged.