Performance: Plasma constructors shallow copy huge Config object many times

plasma.go constructors New(), New2(), New3() all accept argument cfg Config, which is a struct, therefore it is being passed by value not by reference. This means each call, and each child this is passed to that does the same thing, makes a new shallow copy of the entire Config struct. By my count this struct currently contains 113 fields, so these copies are doing a lot of extra CPU work and consuming significant stack.

E.g. a call to New(cfg) triggers the following copies (at least the first 5, and possibly up to 7, assuming none of the great-grandchildren copy it further):

1. New(cfg) – arg copied on call
2. New2(cfg, ...) – copied to child call
3. New3(cfg, ...) – copied to grandchild call
4. applyConfigDefaults(cfg, ...) – copied to first great-grandchild call
5. cfg = applyConfigDefaults(...) – and copied again assigning the returned copy back to New3's cfg variable
6. doRecovery(..., cfg, ...) – copied to second great-grandchild if this call is made
7. newPlasmaSkeleton(cfg, ...) – copied to third great-grandchild if this call is made