JIRA: https://jira.spring.io/browse/INT-4081 * Rename `MessageGroupPreparedStatementSetter` to `ChannelMessageStorePreparedStatementSetter` * Add JavaDocs to the `ChannelMessageStorePreparedStatementSetter` * Polishing `JdbcChannelMessageStore` JavaDocs; remove unused `BeanFactory` * Polishing Docs for typos and proper text flow