Adds minor MVCC improvements
* adds insert performance regression test
* MVCC: If a row is inserted and deleted in the same Tx, it is now invisible to the Tx (as it should be) - fixes #172
* Remove TXManager::getLastCID() calls from loop in Store::Merge #181
* TXManager::commitTransaction should get complete context instead of commit id only - changed rollback as well #181
Merge pull request #194 from hyrise/feature/new_mvcc

Adds minor MVCC updates