usnic: change retrans timeout to 5ms

A bunch of empirical testing has shown that increasing the retranmit
timeout from 1ms to 5ms doesn't adversely affect performance, yet
decreases the number of gratuitious retransmissions.

(cherry picked from commit open-mpi/ompi@c2615a4)

Author: jsquyres

ompi/mca/btl/usnic/btl_usnic_mca.c

@@ -245,7 +245,7 @@ int opal_btl_usnic_component_register(void)
     mca_btl_usnic_component.udp_port_base = (int) udp_port_base;
 
     CHECK(reg_int("retrans_timeout", "Number of microseconds before retransmitting a frame",
-                  1000, &mca_btl_usnic_component.retrans_timeout,
+                  5000, &mca_btl_usnic_component.retrans_timeout,
                   REGINT_GE_ONE, OPAL_INFO_LVL_5));
 
     CHECK(reg_int("priority_limit", "Max size of \"priority\" messages (0 = use pre-set defaults; depends on number and type of devices available)",