Add Database#fetch_size accessor and :fetch_size option to jdbc adapter, for setting JDBC Statement fetch size
Default to a fetch_size of 100 in the jdbc/oracle adapter, similar to the oci8-based oracle adapter
Add Dataset#with_fetch_size to jdbc adapter, for setting fetch size for JDBC ResultSets

This should have the effect of overriding the database's fetch_size.
I'm not sure if the very first fetch will use the database's fetch
size or the dataset's fetch size if both are set, it probably depends
on whether JDBC delays the database fetch until the ResultSet#next call.