Cache values_for in the backend_enum_source to save on DB queries
Don't save the whole related object when we just want to tickle the lock_version
Reuse JSON validators so we don't create one for each validated record
Don't update records that can be linked upon creation instead
Use the new enum caching when looking up values during record creation too
Exception caching for validation
Don't reinitialise each sequence every time we request a value
Don't automatically refresh nested records from the database
Set fragments when reusing validation exceptions
Split ValidatorCache into its own file
Merge pull request #385 from hudmol/perf-tuning-2

Perf tuning 2