BUG/MINOR: stream: Properly handle "on-marked-up shutdown-backup-sessions"

BUG/MINOR: stream: Properly handle "on-marked-up shutdown-backup-sessions"

shutdown-backup-sessions action for on-marked-up directive does not work anymore
since the stream_shutdown() function was modified to be async-safe.

When stream_shutdown() was modified to be async-safe, dedicated task events were
added to map the reasons to shut a stream down. SF_ERR_DOWN was mapped to
TASK_F_EVT1 and SF_ERR_KILLED was mapped to TASK_F_EVT2. The reverse mapping was
performed by process_stream() to shut the stream with the appropriate reason.

However, SF_ERR_UP reason, used by shutdown-backup-sessions action to shut a
stream down because a preferred server became available, was not mapped in the
same way. So since commit b8e3b0a18d ("BUG/MEDIUM: stream: make
stream_shutdown() async-safe"), this action is ignored and does not work
anymore.

To fix an issue, and being able to bakcport the fix, a third task event was
added. TASK_F_EVT3 is now mapped on SF_ERR_UP.

This patch should fix the issue #2848. It must be backported as far as 2.6.