Kill iomux-link jobs on warden server stop

When warden server starts up it recovers jobs and creates new iomux-link
processes. We now kill old process. This will hold the stdout/stderr
data in buffer (64K) until next iomux-link reconnects to it. They use
the same cursor path so it will start from where it was left.

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>
save and recover log tag to/from snapshot

When the Warden server is drained it will now come up and continue
directing job output to syslog if it was initially spawned with that
mode.

[#61998582]

Signed-off-by: Maria Shaldibina <mariash@pivotallabs.com>