tr2tls: clarify TLS terminology

tr2tls: clarify TLS terminology

Reduce or eliminate use of the term "TLS" in the Trace2 code.

The term "TLS" has two popular meanings: "thread-local storage" and
"transport layer security".  In the Trace2 source, the term is associated
with the former.  There was concern on the mailing list about it refering
to the latter.

Update the source and documentation to eliminate the use of the "TLS" term
or replace it with the phrase "thread-local storage" to reduce ambiguity.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>