MINOR: chunk: drop the global thread_dump_buffer

MINOR: chunk: drop the global thread_dump_buffer

This variable is not very useful and is confusing anyway. It was mostly
used to detect that a panic dump was still in progress, but we can now
check mark_tainted() for this. The pointer was set to one of the dumping
thread's trash chunks. Let's temporarily continue to copy the dumps to
that trash, we'll remove it later.

(cherry picked from commit a6698304e03847f3e114b22b4229b382f3ddffd1)
Signed-off-by: Willy Tarreau <w@1wt.eu>