Remove AbortSignal event listeners when requests complete

[paulbrimicombe]

Issue #1318, if available:

Description of changes:

Currently if an AbortSignal is re-used across multiple requests, more and more listeners will accumulate, leading to memory leaks. In order to mitigate this, we should remove any event listeners we add to the AbortSignal when the request completes.

Notes on the code changes

• Should we be more defensive and check that removeEventListener is a function before using it?
• I don't really like the code changes in the fetch-http-handler so if anyone has any suggestions, please do chime in. It's somewhat tricky because fetch doesn't provide an event-driven API (that I know of).

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[paulbrimicombe]

Thanks for approving and merging so quickly @kuhe and @haydenbaker