feat: Ping (#48)

[vrongmeal]

$ s2 pingtest my-basin pingtest/express -n 5
Preparing test...
ack: 28 ms    e2e: 29 ms
ack: 28 ms    e2e: 28 ms
ack: 28 ms    e2e: 28 ms
ack: 27 ms    e2e: 27 ms
ack: 28 ms    e2e: 28 ms

Sent 5 batches with 102402 bytes

-------- Append Acknowledgement Latency Statistics ---------
     Mean 28 ms
   Median 28 ms
      P95 28 ms
      P99 28 ms
      Max 28 ms
      Min 27 ms
  Std Dev 0 ms

-------------- End to End Latency Statistics ---------------
     Mean 28 ms
   Median 28 ms
      P95 29 ms
      P99 29 ms
      Max 29 ms
      Min 27 ms
  Std Dev 0 ms

Resolves: #48

[shikhar]

please factor out a new mod for as much of the new code as reasonable

[shikhar]

@vrongmeal let's just call it ping :)

[vrongmeal]

Updated:

$ s2 ping vr-basin pingtest/express -n 5 -b 50000
Preparing...
50004 bytes:  ack = 30 ms   e2e = 32 ms
50004 bytes:  ack = 29 ms   e2e = 29 ms
49999 bytes:  ack = 28 ms   e2e = 29 ms
49992 bytes:  ack = 29 ms   e2e = 29 ms
50008 bytes:  ack = 29 ms   e2e = 29 ms

Sent 5 batches with 250007 bytes

-------- Append Acknowledgement Latency Statistics ---------
     Mean 29 ms
   Median 29 ms
      P95 30 ms
      P99 30 ms
      Max 30 ms
      Min 28 ms
  Std Dev 0 ms

-------------- End to End Latency Statistics ---------------
     Mean 29 ms
   Median 29 ms
      P95 32 ms
      P99 32 ms
      Max 32 ms
      Min 29 ms
  Std Dev 1 ms

[shikhar]

looks great!