If a transaction is opened and closed without any queries being run, we
can safely omit the `BEGIN` and `COMMIT` statements, as they only exist
to modify the connection's behaviour inside the transaction. This
removes the overhead of those statements when saving a record with no
changes, which makes workarounds like `save if changed?` unnecessary.

This implementation buffers transactions inside the transaction manager
and materializes them the next time the connection is used. For this to
work, the adapter needs to guard all connection use with a call to
`materialize_transactions`. Because of this, adapters must opt in to get
this new behaviour by implementing `supports_lazy_transactions?`.

If `raw_connection` is used to get a reference to the underlying
database connection, the behaviour is disabled and transactions are
opened eagerly, as we can't know how the connection will be used.
However when the connection is checked back into the pool, we can assume
that the application won't use the reference again and reenable lazy
transactions. This prevents a single `raw_connection` call from
disabling lazy transactions for the lifetime of the connection.