Create new span when wrapping executor tasks

[pkoenig10]

When running a task in a wrapped executor, a new span should be created for the task. This will allow tracing information to clearly indicate when the task started and finished running in the executor.

See #59 (comment).

[pkoenig10]

There are actually cases where you don't want this. Specifically when chaining async tasks together. For example:

FluentFuture.from(future)
    .transform(result -> handleResult1(...), wrappedExecutor)
    .transform(result -> handleResult2(...), wrappedExecutor)

Each call to transform will create another nested span because the next callback is being enqueued inside the previous task. In this scenario it would be better to have no executor span to avoid this potentially long chain of nested spans.