[Breaking change for dd-trace-api] Deprecate scope finishOnClose and continuation close

[tylerbenson]

Note: These deprecated methods will be removed in the following release.

The finishOnClose functionality will remain in the OpenTracing compatibility layer, but behavior may change if utilizing on continuations.

Migrating usage can impact the way span duration is calculated. For example given the following code:

@Trace
def someMethod() {
  executor.execute {
    // Assume this executes after someMethod() already returned.
    httpClient.request(url)
  }
}

Historically, the Java Tracer prevented the top level span from finishing until the child work finished, resulting in a trace like this:

[-------------- someMethod --------------------]
                       [---clientRequest-----]

After this change, it will produce a trace more like this, which more accurately depicts the duration of the method execution.

[---someMethod---]   (likely even shorter)
                       [----clientRequest---]

Since trace duration is based on the parent/top level span, this change can impact the trace duration calculation.

[devinsba]

Not related to the change, but maybe the title of the PR should state the module (dd-trace-api) that this is a breaking change for

[tylerbenson]

I'm pulling out all the changes to internal instrumentation into a separate PR.