`lazy_find` should accept lambda to preprocess value

We're hitting an issue where we have to copy value from Flavor to Vm, but divide it by 2^20 prior saving. Something like:

persister.flavors.build(
  ... 
  :memory => flavor.memory_in_mb.megabytes # store in bytes
)

persister.hardwares.build(
  ...
  :memory_mb => persister.flavors.find(flavor_ref).memory / 1.megabytes  # convert to megabytes
)

Problem is that we currently have to use .find() which may potentially result in ordering issues. Would it be possible to let .lazy_find() accept a block where we could preprocess the value? Something like this would be fantastic:

persister.flavors.lazy_find(flavor_ref) { |flavor| flavor.memory / 1.megabyte }

/cc @agrare

Start by locating the implementation and tests for lazy_find, then compare its current behavior with the find examples in this issue. Done means the method accepts the shown block form and applies it to the found flavor value without introducing the ordering problem described.