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.
Unused import (so it seems)