This commit renames `RoleManager` -> `PoolManager` and `Role` ->
`PoolConfig`.

Once we introduced the previous commit, and looking at the existing
code, it's clearer that `Role` and `RoleManager` are not the right names
for these.

Since this PR moves away from swapping the connection handler concepts
around and the role concept will continue existing on the handler level,
we need to rename this.

A `PoolConfig` holds a `connection_specification_name` (we may rename
this down the road), a `db_config`, a `schema_cache`, and a `pool`. It
does feel like `pool` could eventually hold all of these things instead
of having a `PoolConfig` object. This would remove one level of the
object graph and reduce complexity. For now I'm leaving this object to
keep the change churn low and will revisit later.

Co-authored-by: John Crepezzi <seejohnrun@github.com>