p.conf: use global max-reserved=50

with precedence set properly max-reserved is no longer useful for
"load balancing"

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-r: Section: rename _master_directory to _section_directory

avoid confusion by not reusing _master_directory
for master_directory + section

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-s: move section handling into Slave class

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