Adds support for AutoFinishScope

[jam01]

Adds support for AutoFinishScope by checking the type of ScopeManager and using a AutoFinishScope.Continuation when appropriate. Includes some tests.

[sjoerdtalsma]

Since your AutoFinishScope behaviour is optional, could you please make sure not to introduce new runtime dependencies (such as opentracing-util from test to provided instead of runtime) ?

You will have to take care of some gotcha's to make this work:

1. Don't import any optional classes but use their fully qualified name (otherwise the import will throw linkage error when your class is loaded instead of when the optional class is actually needed)
2. Catch and anticipate a linkage error (usually NoClassDefFoundError)

[jpkrohling]

What's the status of this PR?