trace2: destroy context stored in thread-local storage

trace2: destroy context stored in thread-local storage

Each thread may have a specific context in the trace2 subsystem that we
set up via thread-local storage. We do not set up a destructor for this
data though, which means that the context data will leak.

Plug this leak by installing a destructor. This leak is exposed by
t7814, but plugging it alone does not make the whole test suite pass.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>