release 0.50 broke backward compatibility with flush() method

[ToddBradley]

Describe the bug

Release 0.50 broke backward compatibility by removing the flush method on DogStatsd. The library documentation recommends that client apps use flush (see below). So either...

1. It was removed by accident, as part of PR 844 (Add client side aggregation #844), or
2. It was removed on purpose, and nobody thought to mention the breaking change in the CHANGELOG

To Reproduce

Steps to reproduce the behavior:

1. Take any application code that is compatible with version 0.49 and uses DogStatsd and the flush() method
2. Try to build or run it with version 0.50. It fails, because the flush method is missing

Expected behavior

If the design is moving away from wanting client apps to use flush, then a) the method should be marked deprecated, b) a note should be published in the CHANGELOG, and c) the documentation should be updated.

If the change was accidental, then regression tests should have caught the unintentional change and blocked PR 844.

Screenshots

N/A

Environment and Versions (please complete the following information):

datadogpy 0.49 and 0.50

Additional context

The documentation refers to flush here:

• https://datadogpy.readthedocs.io/en/stable/index.html?highlight=flush#datadog.dogstatsd.base.DogStatsd.flush
• https://docs.datadoghq.com/developers/dogstatsd/high_throughput/

See also #858

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