When a unary request takes more than predefined duration, this request
is defined as "expensive" and a warning is printed. The expensive request
duration is hard-coded to 300 ms. It can be not enough for example
for transactions with a lot of operations. The warnings just blow up
the log files and reduce throughput.
This fix allows user to configure the "expensive" request duration.
Signed-off-by: Alexey Roytman <roytman@il.ibm.com>