[#1144] Widen the ephemeral port range before the benchmark runs (#1145)

BIND opens a new connection per iteration, and docker-proxy relays each one to
the container over a second connection from an ephemeral port. At ~500 binds/s
and 60 s of TIME_WAIT that leg outgrows the default range (32768-60999) within a
minute; the proxy then drops new connections and BIND fails with "LDAP
connection has been closed".

Before the benchmarks of compare-opendj.sh and of the OpenLDAP vs OpenDJ
workflow run, set net.ipv4.ip_local_port_range to 1024-65535 and enable
net.ipv4.tcp_tw_reuse.

Fixes #1144