[jdbc] Separate use of JDBC batch update APIs from autoCommit

Tried to simplify the distinction between using JDBC's addBatch()/executeBatch()
API calls and "batching" updates via autoCommit=false and a manual commit() after
a given number of updates.

Breaks out flavors into their own package to reduce bloat in JdbcDBClient.

Encompasses changes from Enis Soztutar.