Replace magic -3 with proper consonstant var
String->long in-memory efficient index for batch importer

for use in cases where the node ids given in the node/relationship input
aren't actual record ids, but arbitrary strings. When using this type of
IdMapping the node ids (strings) are fed to the in-memory index during the
node import stage and sorted, and general prepared for getting
String->long later in other stages that deals with relationships where
they also specify these strings as the start/end node ids.
Merge pull request #3102 from S124568/fix-magic-constant

[master] Replace magic -3 with proper consonstant var
Merge pull request #3110 from tinwelint/2.2-batch-import-external-ids

[2.2] String->long in-memory efficient index for batch importer
Restructuring of store and transaction packages

o.n.k.i.
  nioneo.store --> store.{record,id}
  transaction.xaframework --> transaction.{log,state,command}

And various other classes moved to a more logical place.