fix bug in paths when initialising with existing RDS2 db
don't forget to load filehash in todo sample code
add optimised dbExists method for RDS2

* can do this faster by inspecting environment directly
faster objectFile

* first check cached environment rather than always recalculating path
Merge branch 'rds2' into rds2-fast

* rds2:
  faster objectFile
  add optimised dbExists method for RDS2
  fix bug in paths when initialising with existing RDS2 db