Prevent possible goroutine rescheduling to another P between
Put and Get calls by locking the goroutine to OS thread.

Inspired by the CL 42770.

Fixes #20198.

Change-Id: I18e24fcad1630658713e6b9d80d90d7941f604be
Reviewed-on: https://go-review.googlesource.com/44310
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Austin Clements <austin@google.com>