Configurable number of parallel xfers (system.transfers).

doc/reference.texi

samples/knot.full.conf

src/knot/conf/cf-lex.l

src/knot/conf/cf-parse.y

src/knot/conf/conf.c

src/knot/conf/conf.h

src/knot/ctl/remote.c

src/knot/server/xfr-handler.c

src/knot/server/xfr-handler.h

src/knot/server/zones.c

src/knot/server/zones.h