MONITOR: Create pidfile after responders started
    
    Resolves:
    https://fedorahosted.org/sssd/ticket/3080
    
    Signed-off-by: Lukas Slebodnik <lslebodn@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    (cherry picked from commit d4063e9a21a4e203bee7e0a0144fa8cabb14cc46)
    (cherry picked from commit 12c36167aff5227d1b1f53988a748283dbb292d9)