Avoids some use of synchronized map in PersistenceManager

o Instead of calling PersistenceManager#currentKernelTransaction() when
  asserting the existence of a transaction, call the (now public)
  #getCurrentTransaction())
o KernelStatement is aware of the NeoStoreTransaction it was created with
  so that PersistenceManager can be avoided.

Going forward PersistenceManager should be deleted and each
NeoStoreTransaction should be tied closer to a kernel transaction. This
will be easier when all operations go through the Kernel API.
Merge pull request #1780 from tinwelint/2.0-less-persistence-manager

Avoids some use of synchronized map in PersistenceManager