Change connection mode to asynchronous

The default mode for NNG is to make TCP connections in
a synchronous or blocking manner (opposite of Nanomsg).
This change causes RMR to invoke the NNG connection function
(dialer) with the non-blocking flag set.  This will likely
result in soft failures (retries) on message send attempts
until the connection is completed.

The original blocking mode can be enabled by setting the
RMR_ASYNC_CONN environment variable to 0 which will turn
this off and cause connections to block. Absense of the
variable, or setting it to 1, causes asynch mode to be on.

This addresses the issue described in RICPLT-2215.

Signed-off-by: E. Scott Daniels <daniels@research.att.com>
Change-Id: I4f544d772227b784efd1f7146a18921b91ae7f8e