Adds a couple of options that provide more control over the default
timeouts and logging.
These are exposed to the shaping helper as `publish_timeout`,
`publish_pool_idle_timeout` and `publish_connection_verbose`.
This makes it easier to compose secret management with a variety
of lua functions without having to build that directly into each
of them.
In particular: one can now use vaults or other secret stores that
we add in the future to manage credentials for HTTP clients.
Wez Furlong