Check that original log file exists before trying to copy it.
StoreUpgrader minor cleanup.

o Remove unused msgLog field and constructor parameter.
o Use FileSystemAbstraction instead of unsafe java.io.File operations.
Batch inserter support for labels and schema indexes.

o Creating nodes with labels, adding labels and removing labels.
o Listing labels for a node.
o Creating schema indexes.
o Schema indexes are (re-)populated on batch inserter shutdown.

o Support for all the above, except creating new schema indexes,
  in the Batch Database.