Add a signal for "locked fluid volume"

[jagoly]

Right now, for multi item provider stations, we have the locked slots per wagon signal that can be used for when there might be extra items stuck in inserters. I believe it makes sense to add similar functionality for fluids as well.

Consider a setup like this:

As long as you only have at most one fluid box between pumps, then you can have setups like this one where you can load one of many different fluids into the one wagon (or pair of short wagons in this case) without being limited by the size of your bays.

The issue with this setup is basically the same as with items, except instead of items stuck in inserters, you have the little bit of extra fluid in the pipes. There is no way to work around this currently that doesn't have major drawbacks.

As a solution, I propose adding a new "Locked volume per fluid wagon" signal, which does exactly what you would expect. In conjunction with the inactivity condition, this would solve the problem cleanly with fluid wagons of any size.