MINOR: tinfo: keep a copy of the pointer to the thread dump buffer

MINOR: tinfo: keep a copy of the pointer to the thread dump buffer

Instead of using the thread dump buffer for post-mortem analysis, we'll
keep a copy of the assigned pointer whenever it's used, even for warnings
or "show threads". This will offer more opportunities to figure from a
core what happened, and will give us more freedom regarding the value of
the thread_dump_buffer itself. For example, even at the end of the dump
when the pointer is reset, the last used buffer is now preserved.

(cherry picked from commit 6d8a523d145a559d2ba4a14b46d27c73fe914c15)
[wt: dropped the doc which is not present in 3.1]
Signed-off-by: Willy Tarreau <w@1wt.eu>