derive Debug for Pool

[onsails]

In some frameworks Debug is required to put Pool in app state's context structs which define Debug by default.

[abonander]

We'd love to accept this change, but we'd like to manually impl Debug for Pool to only show information that's meaningful to the user instead of deriving for Pool and SharedPool. For one, we don't need to require Debug for the DB type parameter which the derive would.

The manual impl should print the following values as if they were fields of Pool using Formatter::debug_struct():

• SharedPool::url as url (needs a getter that Pool can access)
• SharedPool::size() as size
• SharedPool::num_idle() as num_idle
• SharedPool::closed as closed (needs a getter)
• SharedPool::options() as options (we can leave the derive on struct Options)

[onsails]

Thanks for feedback, updated PR.

[abonander]

Great! Thanks for the quick response. I'll merge this when the build's passing.