Created an interface for TracingConnection, to allow checking whether a connection is wrapped.

[RutgerLubbers]

Created an interface for TracingConnection, renamed old class (with same name) into TracingConnectionWrapper.

This allows checking whether a connection has already been wrapped. And prevents an exception with (at least) the Oracle JDBC driver, where the isWrapperFor method only accepts interfaces.

After this opentracing-contrib/java-spring-cloud should be adepted to use this code.

[coveralls]

Pull Request Test Coverage Report for Build 239

• 27 of 100 (27.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 46.432%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/io/opentracing/contrib/jdbc/TracingDataSource.java	1	2	50.0%
src/main/java/io/opentracing/contrib/jdbc/TracingConnectionWrapper.java	25	97	25.77%
<!--	Total:	27	100	27.0%	-->

Totals
Change from base Build 238:	0.0%
Covered Lines:	449
Relevant Lines:	967

---

💛 - Coveralls

[malafeev]

@RutgerLubbers if TracingConnection is created as proxy:

Connection tracingConnection = WrapperProxy
        .wrap(connection, new TracingConnection(connection, connectionInfo, withActiveSpanOnly,
            ignoreStatements, tracer))

Does it have the same issue? Creating proxy connection is more correct way than direct constructor to avoid issues with casting to real connection class. cc @safris

[malafeev]

not needed