Previously, it was very easy to accidentally leak a database password in
production logs if an error ends up calling inspect on a ConnectionPool
or an individual connection (Adapter). This is due to the default
`#inspect` output for Pools and Adapters being unnecessarily large, and
both currently including passwords (through the DatabaseConfig of a
Pool, and the internal configuration of an Adapter).

This commit addresses these issues by defining a custom `#inspect` for
ConnectionPool, AbstractAdapter, and DatabaseConfig. The condensed
`#inspect` only includes a few valuable fields instead of all of the
internals, which prevents both the large output and passwords from being
included.