Print raw symbols in backtraces as a fallback (important for OS X)
Closes #1682.
Revert "Changed debug-mode coroutine accounting to work with thread switches."

This reverts commit ab625b240c49a789c89a41d9bb9a41ca7d692ae2.
Merge branch 'daniel_1681' into next
Merge branch 'next' of github.com:rethinkdb/rethinkdb into next