Much faster serializing with compression and xdr off
Add notes to TODO re optimisation
Merge branch 'rds2' into rds2-fast

* rds2:
  fix bugs in insertion and deletion to objectlist envir
  Switch to use an environment to keep track  of objects
Merge branch 'rds2' into rds2-fast

* rds2:
  simpler length for RDS2
  Remove object list error from TODO
don't forget to load filehash in todo sample code
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
a few more notes in TODO about performance
Add compression and xdr slots for filehashRDS2 class