Trying to derive `Clone` automatically for `PoolOptions` results
in errors when `clone` is actually called. This is because the
derive incorrectly determines that `Clone` is _not_ derivable
due to the lack of `Clone` implementation on the `DB: Database`
type parameter, even though no value of that type is actually
stored in a to-be-cloned position (in fact, it's only used for
the `Connection` associated type on the type parameter's
`Database` trait impl).

Manually implementing `Clone` side-steps this issue and insures
the type is always actually cloneable.

For reference: #2548