osc/rdma: check for more types of window access violations

This commit adds a check to see if the target is in an access epoch. If
not we return OMPI_ERR_RMA_SYNC. This fixes test_start3 in the onesided
test suite. The cost of this extra check is 1 byte/peer for the boolean
flag indicating that the peer is in an access epoch.

I also fixed a problem where mupliple unexpected post messages are not
correctly handled.

cmr=v1.8.2:reviewer=jsquyres
mpit: update the return code check for mca_base_var_get

mca_base_var_get now can return OPAL_ERR_NOT_FOUND if a variable no
longer exists. This commit updates the return code check to ensure
the correct MPI_T error code is returned to the user.

cmr=v1.8.2:reviewer=jsquyres