Adds ability to dynamically find auto-increment id.

Previously, when persisting entities, we always start the
auto-incrementing id at 0, regardless of the entities already in the
table. This commit changes allows us to take the max value for the id column
of a table when finding the autogenerated id to persist.
According to the javadoc...

"A ResultSet object is automatically closed when the Statement object that generated it is closed"

We close the statement in ```JdbcUtil#execute```'s finally block.