DataFetcher#cancel() should be called on a background thread

[sjudd]

OkHttp and HttpUrlConnection both trigger strict mode violations when cancel is called on the main thread (See #257, and the TODO in HttpUrlFetcher).

[TWiStErRob]

@sjudd this seems to be fixed on your branch, did you forget to merge it?

[sjudd]

I tried, but it causes issues with OkHttp. At least that (now relatively old) version of OkHttp seemed to get into a bad state after a few async cancellations of in process requests.

[floating-cat]

OkHttp fixes this issue in v2.7.0.
OkHttp now detects and warns on connections that were allocated but never released, and will enforce HTTP/2 stream limits. This update also fixes Call.cancel() to not do I/O on the calling thread.

https://github.com/square/okhttp/blob/master/CHANGELOG.md
square/okhttp#1592

[TWiStErRob]

@floating-cat thanks! Glide master is currently on okhttp-2.3.0.
@sjudd If we update it to 2.7.0 and add the cancel call in the integration lib, we can fix #257, but not this issue. Some hacky version detection may also be needed to no try to cancel when an older version of OkHttp is forced on the classpath.

[sjudd]

Obsolete with okhttp3 and v4