runtime/pprof: use more efficient hash table for staging profile

runtime/pprof: use more efficient hash table for staging profile

The old hash table was a place holder that allocates memory
during every lookup for key generation, even for keys that hit
in the the table.

Change-Id: I4f601bbfd349f0be76d6259a8989c9c17ccfac21
Reviewed-on: https://go-review.googlesource.com/37163
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>