Coders should have meaningful equals methods

`StandardCoder` implements an equality check based on the component coders and equal classes. Any coder that is configured, or that does not extend `StandardCoder`, should have meaningful implementations of `equals` and `hashCode`

Imported from Jira [BEAM-1202](https://issues.apache.org/jira/browse/BEAM-1202). Original Jira may contain additional context.
Reported by: tgroh.

Start with StandardCoder and trace coder implementations that are configured or do not extend it, noting how component coders and equal classes currently participate in equality. Define the affected coder set and verify that each has meaningful equals and hashCode behavior with focused Java tests.

Written by the indexing model from the issue text.