Fix concurrency issues while overwriting tracefiles in live

Introduce a new lock to set the abort_flag in the viewer stream. This
lock is nested inside the tracefile rotation of the writer and is
taken before reading the tracefile in the viewer-side. That way, the
viewer-side has just to check this flag to know if it is safe to read.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>