INT-2562: JPA: Add 'flush' Operation

JIRA: https://jira.springsource.org/browse/INT-2562

INT-2562: Add 'flush-size' & 'clear-on-flush' attrs

INT-2562: Documentation

INT-2562: Polishing

INT-2562: Addressing PR's comments

INT-2562: Fix typos and JavaDocs