Trace exception local variables and args per frame

New configuration options and defaults
config.locals.enabled = off
config.locals.max_trace_frames = 100

Turned off by default because this may not be performant just yet!

  1. Stores each Ruby process' local vars and method arguments
  2. When gathering exception data, "replay" over each frame to get the variables for that frame's context
  3. Dump the cache and repeat
I think there will normally be one Rollbar instance per process, but not one instance per thread. So if you're using a multithreaded webserver, I think it's possible for the threads to be stepping on each other.

I want to say we've had bugs caused by this before... can't remember off-hand.