thread safety

[ghazel]

Oink is currently not thread safe. The use of class variables in the ActiveRecord instrumentation allows for two threads to step on each other. Instead, Thread.current['some_key'] should be used.

[noahd1]

Think this commit addresses the issue you raised, though I'm not convinced that oink is completely thread-safe otherwise. I haven't tried running oink in a multi-threaded environment. One other piece of global state I'm aware of is "@oink_extended_active_record" which is accessed/set during middleware instantiation.