BUG 2734 : Use the 95th percentile number to determine backpressure

When running tests with OpenFlow I see the following transaction completion
times consistently for 127 and 255 switches with statistics on.

50% - 5-6 ms
75% - 7-8 ms
95% - 20-60 ms
98% - 100-160 ms

By using the 95th percentile number to determine transaction creation backpressure
we can safely increase our throughput without causing timeouts due to outliers.

Change-Id: I0d9739b3045afb389583f2dd18ac431215a28f84
Signed-off-by: Moiz Raja <moraja@cisco.com>