Properly dispose CancellationTokenSource

[khellang]

This fixes a memory leak of 40 bytes per request, introduced in 4aeaf31.

Because of the nesting (in the using block), this is best reviewed with ?w=1 😉

We need to push this as 1.4.2 ASAP.

Closes #2113.

[khellang]

I assume Dispose will do The Right Thing™ and cancel before cleaning up...

[khellang]

Before

$ wrk -t12 -c400 -d30s http://10.211.55.3:8888/nancy/
Running 30s test @ http://10.211.55.3:8888/nancy/
  12 threads and 400 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   865.58ms  255.22ms   1.86s    71.11%
    Req/Sec    26.59     18.56   120.00     74.73%
  8237 requests in 30.10s, 10.02MB read
  Socket errors: connect 155, read 0, write 0, timeout 0
Requests/sec:    273.64
Transfer/sec:    340.98KB

After

$ wrk -t12 -c400 -d30s http://10.211.55.3:8888/nancy/
Running 30s test @ http://10.211.55.3:8888/nancy/
  12 threads and 400 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   866.75ms  255.64ms   1.99s    71.21%
    Req/Sec    26.96     18.93   110.00     71.71%
  8208 requests in 30.09s, 9.99MB read
  Socket errors: connect 155, read 0, write 0, timeout 0
Requests/sec:    272.78
Transfer/sec:    339.91KB

[damianh]

👍 @khellang