p-m: remove obsolete and unused parts

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: use a persistent master connection

Keep the master connection open to reduce the number of ssh connections
opened. Close the connection after errors, before processing packages, or
before going to sleep.

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s.deb: restrict slave's ssh key to only allow running piuparts-master

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: support empty master command

the command may be set in authorized_keys

Signed-off-by: Andreas Beckmann <anbe@debian.org>