btl_usnic_mca.c: Increase default connectivity checker frequency

In abusive MPI communication patterns, sending a UDP ping only once a
second may not be sufficient -- all the UDP pings may be dropped.  So
increase the frequency of the pings to every quarter second, and allow
more total pings to be sent.

Total timeout time is still the same (10 seconds) -- we'll just now
try 40 times (i.e., once every quarter second) as opposed to 10 times
(i.e., once a second).  Testing has shown that this frequency allows
the connectivity checker to always succeed even in the many-to-one
abusive communication patterns.

cmr=v1.8.2:reviewer=dgoodell