improve push performance

To make it work:
- need to append &amp;rewriteBatchedStatements=true to mysql connection url
- ~~need to change entity id generation strategy to Table~~
- need to add a few hibernate properties

Point 1 needs to be performed manually (edit zanata-ds.xml or standalone.xml if in openshift). Point ~~2~~ and 3 are done in this PR

**note**
turn out we can not use table ID generator in JBoss(managed environment). Quoted from MultipleHiLoPerTableGenerator javadoc
> A hilo IdentifierGenerator that returns a Long, constructed using a hi/lo algorithm. The hi value MUST be fetched in a seperate transaction to the Session transaction so the generator must be able to obtain a new connection and commit it. Hence this implementation may not be used when the user is supplying connections. In this case a SequenceHiLoGenerator would be a better choice (where supported).

The problem is we get connection from datasource therefore it can't guarantee it to be a new connection and it may fail with exception "Caused by: java.sql.SQLException: You cannot commit during a managed transaction!"

Therefore batch insert is not possible with mysql. Batch update can be achieved.
A couple of questions there: Does Mysql have a limit on overall query size? If so, then it might also affect this method, even if it doesn't limit the number of elements in the IN clause.

But most importantly, since this is used for batching wouldn't it be better to get ... well, 'batches'... of text flows for a document, using their position as the batch marker?