Instead of creating a type for each `TaskSource` variety have each `TaskSource`
hold the same kind of sender (this was inconsistent before, but each
sender was effectively the same trait object), a pipeline, and a
`TaskSourceName`. This elminates the need to reimplement the same
queuing code for every task source.
In addition, have workers hold their own `TaskManager`. This allows just
exposing the manager on the `GlobalScope`. Currently the `TaskCanceller`
is different, but this will also be eliminated in a followup change.
This is a the first step toward having a shared set of `Sender`s on
`GlobalScope`.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
