This is made especially to counter problems with RDMA over omni-path. It turned out that
rdma_route_resolve() fails sometimes when there are massive amounts of connections opened
at the same time (in our case the problem starts when opening several hundred connections at
once, for example when using a benchmarking tool). This is kind of nondeterministic, so it doesn't
happen all the time.
The introduced option limits the amount of parallel connection attempts so this doesn't happen
anymore. There is no recommended value since it's hard to find out where it stops working.
A value of 10 to 100 seems to be pretty good though, the connection process even seems to be faster.
(cherry picked from commit 7ab3302c4447d1110ab30dc1dbf178e5855212f4)