Host proxy object

[mikkokar]

Adds a new routing object: HostProxy. It proxies traffic to single configured endpoint.

It supports:

• Styx connection pool (with expiring connections)
• TLS settings
• Response timeout configuration
• Origin/Application and connection pool metrics.
• A metric prefix for backwards compatibility

The following "features" are omitted from this pull request:

• Request logging at styx client level. This shouldn't be necessary as logging could be implemented as a separate routing object.
• Origin/Application metrics are not de-registered after the host proxy object is removed. Styx application routing at present doesn't de-register them anyway, and therefore we'd have to implement this as a separate increment.