[EchoClient] support latency histograms

Histograms can only be collected when using a fixed amount of iterations.

When the '--histogram <file>' argument is passed each Client
collects 4 time stamps (each 8 byte):

1. Before requesting the send operation
2. After requesting the send operation
3. After getting unblocked and dispatched because the send operation finished
4. After getting unblocked and dispatched because the recv operation finished

Taking the timestamps is enabled using a template and thus does not introduce
any runtime cost if they are not used except binary size.

Before termination three latencies are calculated and written to the histogram
file as csv data for each client and each echo.

1. total_latency := (T4 - T1)
2. after_send_latency := (T4 - T2)
3. after_send_dispatch_latency := (T4 - T3)