Client write buffers

[mochi-co]

Following our discussion in #150 and #160

This PR removes the fixed pool in exchange for writing directly to the clients via client-specific buffered write channels. In the event that a client's write buffer is full, in such an event as a slow reading client, the message is dropped and the new OnPublishDropped hook is called. If the message was QoS 1 or 2, the inflight is rolled back and the message is treated as unattempted.

Also corrects tests and updates benchmarks. Paho is passing.

cc @thedevop thank you for your insights! If you could take a look and see if this PR is appropriate, I would be very grateful :) My apologies for the delay!

[coveralls]

Pull Request Test Coverage Report for Build 4138174316

• 45 of 46 (97.83%) changed or added relevant lines in 4 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.08%) to 98.587%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
server.go	22	23	95.65%

Files with Coverage Reduction	New Missed Lines	%
server.go	3	99.62%

Totals
Change from base Build 4137585804:	-0.08%
Covered Lines:	5094
Relevant Lines:	5167

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💛 - Coveralls

[thedevop]

Looks good, just one inherited bug that's not related to this change.