MINOR: spoe: Support the async mode with several threads

A different engine-id is now generated for each thread. So, it is possible to
enable the async mode with several threads.

This patch may be backported to older versions.

(cherry picked from commit b1bb1afa4741a20e5bf954f0065ae7b747a3e219)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
2 files changed