Change public DataChannel props to methods

[enobufs]

As discussed in slack, all DataChannel public props are converted to methods to avoid race conditions.

Resolves #505

[coveralls]

Pull Request Test Coverage Report for Build 2544

• 100 of 103 (97.09%) changed or added relevant lines in 3 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.4%) to 81.117%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
datachannel.go	93	96	96.88%

Files with Coverage Reduction	New Missed Lines	%
datachannel.go	1	76.61%

Totals
Change from base Build 2539:	0.4%
Covered Lines:	4184
Relevant Lines:	5158

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💛 - Coveralls