Historically, OpenSSL has used callbacks for anything thread related,
but we don't actually have that many threading libraries to worry about:
just pthreads and Windows (I hope).

That suggests that it's quite reasonable to handle threading ourselves,
and eliminate the need for users to remember to install the thread
callbacks.

The first user of this would be ERR, which currently simulates
thread-local storage using a lock around a hash table keyed by the TID.
(Although I suspect that change will need some CMake work in order that
libpthread is automatically included with libcrypto when linking tests
etc, but not on Windows and without lots of ifs.)

Change-Id: I4dd088e3794506747f875c1f3e92b9bc6700fad2
Reviewed-on: https://boringssl-review.googlesource.com/4010
Reviewed-by: Adam Langley <agl@google.com>