Close down okhttp dispatcher executor

[rouzwawi]

This seems to be the way one should close the okhttp client according to: https://github.com/square/okhttp/blob/master/okhttp/src/main/java/okhttp3/OkHttpClient.java#L90-L100

I've tested this with a CLI that earlier would hang the JVM for about 30 seconds before actually exiting. Looking at the threads, I saw that one non-deamon thread called Dispatcher was not stopped.

[codecov-io]

Current coverage is 66.07% (diff: 100%)

Merging #137 into master will decrease coverage by 1.60%

@@             master       #137   diff @@
==========================================
  Files           137        123    -14   
  Lines          2813       2550   -263   
  Methods           0          0          
  Messages          0          0          
  Branches        237        223    -14   
==========================================
- Hits           1904       1685   -219   
+ Misses          862        819    -43   
+ Partials         47         46     -1   

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[spkrka]

👍

[mattnworb]

this will be very helpful, thanks @spkrka

[spkrka]

@mattnworb all credit goes to @rouzwawi , I just reviewed it. :)

[mattnworb]

d'oh I meant to comment on #135 !