Stream cleanup

[DanSimon]

There are several significant changes in this PR focused on cleaning up and generalizing some of the http streaming code:

• Began moving some streaming-related files into a new streaming sub-folder. I will have another PR that moves some other files containing unchanged code.
• The StreamServiceController has been generalized into StreamTranscodingController. Not only can this class now be used with streaming protocols besides http, but it can work for both server and client connections. This definitely took a little type trickery to get all the parameters to behave with http, but it is all hidden from end-users. This should also work as the multiplexing layer for http2
• New StreamingHttpClient. Unlike other clients, there will be no Future version of this client, since at least for now streams are not thread-safe.
• A new BaseHttpMessage[H,B] type that can serve as a base trait for all http protocols, including (eventually) http2. This opens the door for being able to do write more generic types and operations on http, http-stream, and streaming-http messages.
• A new filterMap typeclass operation on sources, used by streaming http as part of the decoding process.
• Few small bug fixes encountered along the way

All existing tests pass. I may add a few more if test coverage is too low.

[codecov-io]

Current coverage is 84.67% (diff: 88.00%)

Merging #489 into develop will increase coverage by 0.40%

@@            develop       #489   diff @@
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  Files            91         93     +2   
  Lines          4208       4248    +40   
  Methods        3859       3888    +29   
  Messages          0          0          
  Branches        349        360    +11   
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+ Hits           3546       3597    +51   
+ Misses          662        651    -11   
  Partials          0          0          

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