Concurrent use of tls.Config is allowed, and may lead to
KeyLogWriter being written to concurrently. Without a mutex
to protect it, corrupted output may occur. A mutex is added
for correctness.

The mutex is made global to save size of the config struct as
KeyLogWriter is rarely enabled.

Related to #13057.

Change-Id: I5ee55b6d8b43a191ec21f06e2aaae5002a71daef
Reviewed-on: https://go-review.googlesource.com/29016Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>