Move sequence value methods to Model level

`prefetch_primary_key?` and `next_sequence_value` methods live in the
connection level at the moment, that make sense when you are generating
the sequence from the database, in the same connection. Which is the use
case today at the Oracle and Postgres adapters.
However if you have an service that generates IDs, that has nothing to
do with the database connection, and should not be fetched from there.
Another use case, is if you want to use another connection to fetch IDs,
that would not be possible with the current implementation, however when
we move those methods to the model level, you can use a new connection
there.

Also this makes easier for gems to add behavior on those methods.

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