Handle timeout on duplex call

Previously, clnt_vc_call wasn't returning RPC_TIMEDOUT on duplex calls
that timed out.

Signed-off-by: Adam C. Emerson <aemerson@linuxbox.com>
Use realtime clock for cond_timedwait timeouts

This should get rid of some of the spurious timeouts. For improved
performance we use CLOCK_REALTIME_FAST (which is defined to
CLOCK_REALTIME_COARSE on Linux.)

Signed-off-by: Adam C. Emerson <aemerson@linuxbox.com>