Improve the design and extensibility of the hosting service HTTP support.

Review Request #1453 — Created Nov. 10, 2018 and updated

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The hosting service HTTP support has always been little more than a
wrapper around urllib2, with `HostingServiceClient` a loose wrapper
around that. Over time, we bolted on new specializations, adding things
like JSON versions of the HTTP methods to the client, working around the
design to allow for Digest Auth and better error reporting in Gerrit,
and trusting that string types were being passed around correctly.

This change takes us a couple steps forward toward a better design
that's less directly dependent on urllib2 (though it still does require
it, and it would take more work to remove it fully).

`URLRequest` has been renamed to `HostingServiceHTTPRequest`, and no
longer subclasses `urllib2.Request`. Instead, it keeps all the state it
needs for the reuqest and then builds a `urllib2.Request` when
performing the request, turning the result into a
`HostingServiceHTTPResponse`.

The response object contains the response URL, payload contents,
headers, and HTTP status code. It also contains a `json` property that
will attempt to deserialize the payload contents as JSON, which replaces
the need for the `json_*()` wrappers in `HostingServiceClient` (all of
which are now deprecated).

The response object can be treated like a tuple, returning the response
data and headers, in order to emulate the behavior of the old `http_*()`
and `json_*()` methods. This behavior is considered deprecated.

It also type-checks the payload data and headers, logging and raising
exceptions if they don't contain byte strings. This is designed with
Python 3 compatibility in mind (though this still needs real testing, as
more of the codebase is made compatible). The intent is to help catch
issues in unit tests, which it already has.

`HostingServiceClient` itself has new methods to better construct
requests and process responses.

First off, clients can now turn on/off support for HTTP Basic Auth and
Digest Auth through flags on the client, which means we can now remove a
lot of logic from Gerrit.

`HostingServiceClient.http_request` now builds a
`HostingServiceHTTPRequest` through `build_http_request()`. This passes
the request arguments to an instance of the class and then adds auth
headers, based on the above flags. Subclasses can override the building
behavior if they need anything custom (and can specify a subclass of the
request class through the `http_request_cls` attribute).

The request is then fed into `open_http_request()`, which is a thin
wrapper around `HostingServiceHTTPRequest.open()` (mostly useful for
unit testing, but subclasses can customize opening behavior as well).
If successful, the result is processed in `process_http_response()`,
which by default will just return the response (subclasses can add
additional logic here, such as rate limiting header checks). If the
request is not successful, the error is passed to
`process_http_error()`, where it can be handled specially by the hosting
service.

Except for specialized services (like Gerrit, which had to be updated in
this change) or in unit tests where types were wrong, this is
backwards-compatible with most existing code.  Future changes will
update more hosting services to benefit more from these changes,
simplifying logic.

Testing Done:
Unit tests pass on Python 2.7 (and 3.x along with upcoming changes).

Tested some standard operations on a few repositories. Didn't see any
breakages.

Reviewed at https://reviews.reviewboard.org/r/10212/


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