Replace DefaultServerCapabilities with NewDefaultServerCapabilities()…

[thedevop]

… to avoid data race

In tests, Capabilities maybe altered. However, DefaultServerCapabilities is a single global instance, where it maybe embedded in running server instances. So when value is altered, tests may fail with data race issue.

Here is an example: In TestServerProcessSubscribeACLCheckDenyObscure: https://github.com/mochi-mqtt/server/blob/65c78534dcb5861dc8ea2163ac58ce603b6e9881/server_test.go#L2790-L2794 and TestServerClearExpiredRetained https://github.com/mochi-mqtt/server/blob/65c78534dcb5861dc8ea2163ac58ce603b6e9881/server_test.go#L3328 are seemingly different tests, but they cause interferences and resulted in data race issue.

This surfaced in the failed test for PR #359.

Note, this is mostly a test issue, as normally one would (should) not alter server capabilities once server started serving.

This may also be the cause for data race issue seen in #200.

[coveralls]

Pull Request Test Coverage Report for Build 7773058128

• 0 of 18 (100.0%) changed or added relevant lines in 1 file are covered.
• 6 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 98.648%

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Files with Coverage Reduction	New Missed Lines	%
server.go	6	99.14%
<!--	Total:	6		-->

Totals
Change from base Build 7495506564:	-0.1%
Covered Lines:	5545
Relevant Lines:	5621

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💛 - Coveralls

[mochi-co]

This is a very nice fix, thank you @thedevop. I really like what you've done here.