feat(agent): add `close` method

[JKRhb]

This PR adds a close method to the Agent class that aborts all pending requests and resolves #312. To do so, a small bug is fixed in the abort method where the _msgInFlight hasn't been decremented when a request is cancelled.

An additional test is added to assert that the new method works as expected.

[coveralls]

Pull Request Test Coverage Report for Build 1473967935

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.9%) to 88.312%

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Totals
Change from base Build 1427777898:	0.9%
Covered Lines:	562
Relevant Lines:	610

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💛 - Coveralls

[JKRhb]

Ping @relu91 :)

[JKRhb]

I'm not coap expert but looking at the logic seems good. This should allow freeing all resources without waiting for responses :)

Thank you for the feedback! :) I added a last couple of changes to the method so that you can both provide an optional callback and listen for close events. This way the API is more similar to the one the CoAP server provides. I think if @Apollon77 could also have a look on it then the PR should be able to be merged :)

[Apollon77]

Add that close before 1.0 or after? ;-)

[Apollon77]

BTW: LGTM

[JKRhb]

Add that close before 1.0 or after? ;-)

Hmm, good question ;) I think we could already integrate it in 1.0 if this would be fine for you, but we could also put it into a separate minor (or patch?) release if you prefer.

[Apollon77]

:-)