Fully support dynamic linking

[davidblewett]

Currently, it is possible to dynamically link against librdkafka at runtime. However, if there are symbols in the new lib that aren't present in rdkafka-sys::bindings things are left in an inconsistent state. It seems possible to make that module be a build artifact instead of static. I plan on spending some time to see if that is doable, but wanted to see if there's resistance to that or something I'm not seeing?