Ensure we don't silently fail when unable to make a connection - bark pleasantly first.

Refs #4571
comm: detect if we run out of communicator ids (cids)

Due to a leak in the osc/rdma component we were running out of cids on
a one-sided tests. This resulted in a hang instead of an error. This
commit causes the nextcid algorithm to return an error if we run out
of cids.

cmr=v1.8.2:reviewer=jsquyres
osc/rdma: do not leak the receive request

This commit fixes a bug that can cause request and communicator leaks
when cleaning up an OSC window. The should prevent a hang seen with
IMB-EXT.

cmr=v1.8.2:reviewer=jsquyres