Auto detect ConnectInfo from underlying DataSource

[quaff]

Most popular DataSource provide method to get jdbc url, we should use it.

[coveralls]

Pull Request Test Coverage Report for Build 299

• 9 of 15 (60.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.07%) to 48.206%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/io/opentracing/contrib/jdbc/TracingDataSource.java	9	15	60.0%
<!--	Total:	9	15	60.0%	-->

Totals
Change from base Build 298:	0.07%
Covered Lines:	524
Relevant Lines:	1087

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💛 - Coveralls

[quaff]

@malafeev How about this.tracer = tracer != null ? tracer : GlobalTracer.get() and add a minimal constructor TracingDataSource(DataSource underlying)? I will create a new PR if this merged.