Fix concurrent map access for clients and inflights causes data race

[mochi-co]

As per #98 and #96 - a data race was occurring when the primary server routine and client routines attempted to access the client inflight messages map at the same time, causing fatal errors.

The issue was reproduced placing the server under heavy load with inovex/mqtt-stresser, using qos=2 for both the publisher and subscriber (see #98).

This PR fixes the issue by returning reference maps of the original values, values in the case of inflight messages, pointers to clients in the case of the client map.

• The inflight messages are never modified by reference once set, so it is reasonable and more cost effective to copy the whole value.
• The clients are passed by reference, and have more sophisticated locking mechanisms and isolation.

Profiling the solution before and after show a small but negligible reduction in mallocs. When running the above mentioned stress tests, data races or crashes are detected are no longer detected.

With many thanks to @muXxer for identifying the issue and providing a potential solution in #96 :)

Additionally, this PR increases the size of the inline publishing buffer in light of #95.
Finally, error message var naming in clients_test has been updated to better meet code standards.

[coveralls]

Pull Request Test Coverage Report for Build 2981567678

• 13 of 13 (100.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.04%) to 97.843%

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Files with Coverage Reduction	New Missed Lines	%
server/server.go	1	96.64%

Totals
Change from base Build 2878940102:	0.04%
Covered Lines:	2767
Relevant Lines:	2828

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💛 - Coveralls