impl: global registration context

[qzhuyan]

intro. global registration context.
Use c_ctx wrap global registration handle.

prepare to get the list of listeners/connections under registration.

[coveralls]

Pull Request Test Coverage Report for Build 6315416084

• 91 of 116 (78.45%) changed or added relevant lines in 7 files are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.05%) to 83.033%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
c_src/quicer_listener.c	6	7	85.71%
c_src/quicer_connection.c	8	11	72.73%
c_src/quicer_nif.c	25	29	86.21%
c_src/quicer_reg.c	48	65	73.85%

Files with Coverage Reduction	New Missed Lines	%
c_src/quicer_config.c	1	83.5%
c_src/quicer_listener.c	1	84.78%

Totals
Change from base Build 6297714086:	-0.05%
Covered Lines:	3137
Relevant Lines:	3778

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💛 - Coveralls