CLEANUP: Make parameters of srv_downtime and srv_getinter const

The parameters of srv_downtime and srv_getinter are not modified
and thus may be const.

Signed-off-by: Simon Horman <horms@verge.net.au>
CLEANUP: Remove unused 'last_slowstart_change' field from struct peer

This was inadvertently added by "MEDIUM: checks: Add agent health check".
It appears to have never been used.

Signed-off-by: Simon Horman <horms@verge.net.au>