Improve /flush handler

[joe-elliott]

Is your feature request related to a problem? Please describe.
The /flush handler on the ingesters will correctly force flush all traces to the backend, but it will not stop the ingester from receiving anything new. We should provide a parameter that allows the user to both flush and force the ingesters to stop accepting traffic. This will be required when performing certain operational procedures like scaling down ingesters.

Describe the solution you'd like
The flush handler should be extended to take a parameter that causes the ingester to:

• Remove itself from the ring
• Stop ingesting new traces
• Force flush everything to the backend