Skip identical backtraces from "re-raised" exceptions

[databus23]

Using sentry in a rails application I noticed that recorded exception contain a lot of duplicated backtraces blowing up the submitted JSON unnecessary.

This happens because in the great big rails onion there are a lot of places where exception are rewrapped preserving the backtrace.
E.g. all errors in views get wrapped in an ActionView::Template::Error

Depending on used gems and middleware the JSON payload gets quite big.
I'm seeing reported exceptions with 300KB of json containing 9 exceptions but only three distinct stacktraces.

I'm not sure if this PR is the best solution for tackling this but I wanted to have this elicit some feedback on how this could be improved.

[dcramer]

@nateberkopec thoughts on implementation? For correctness I realize this isn't great, but from a usability point of view I def agree with it. The approach to me seems pretty ok. We can def maintain the list of exceptions, and just not dupe the stacktrace.