Use no custom header APIs when header type isn't used

[alzimmermsft]

Description

Updates many service client APIs to use the NoCustomHeaders generated API. The NoCustomHeaders variant doesn't convert the response HTTP headers into a typed object, the service client APIs moved to these variants didn't use the custom headers being returned by the non-NoCustomHeaders variant. Aka, they were wasting CPU cycles on converting the HTTP headers to a Java type just to throw away that work. This should provide a performance improvement.

Additionally, the thread pool being used internally by Storage SDKs was replaced by SharedExecutorService in azure-core. This is preferable as this is meant to be an ExecutorService shared by all Azure SDKs so that each SDK doesn't need to configure, and possible expose ways to modify the behavior, their own ExecutorServices. Long term this will allow for azure-core to simply provide better integration into application environments such as allowing the SharedExecutorService to use an ExecutorService provided by the application rather than one in the implementation details of azure-core (allowing customers to better manage how the Azure SDKs use resources).

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[azure-sdk]

API change check

API changes are not detected in this pull request.

[alzimmermsft]

/azp run java - storage - tests

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[alzimmermsft]

/check-enforcer override