Allow persistent=false to be set externally

[zarqman]

This PR allows connection.persistent = false to be called externally.

Use cases:

1. Server connections - As part of a more graceful shutdown, I want in-flight connections to be allowed to finish the current request/response, but then terminate and not be reused. In addition to closing the connection, flagging connections in this way allows them to return a Connection: closed header so the client is properly notified that this connection isn't to be reused for further requests.

2. Client connections - When certain conditions are met, I want to flag client connections as not eligible to be returned to the Pool for reuse. This could be because the connection configuration has changed, a maximum request count has been reached, etc.

Alternatives

I'm not sure setting connection.persistent = true is ever warranted. As such, I considered adding a #not_persistent! method instead that only sets to false. I chose to go with the simpler solution of an accessor, but going with something more opinionated is an option if preferred.

Types of Changes

• New feature.

Contribution

• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.

[ioquatix]

I think we should expand on the documentation a little and also add a warning about setting persistent to true when the other end is not expecting it.

[zarqman]

Updated!