RFR: update to lambda metafactory spec

[Brian Goetz]

This webrev:

   http://cr.openjdk.java.net/~briangoetz/JDK-8019646/webrev/

contains minor changes to the lambda metafactory implementation, and a 
significant overhaul of its specification.  There are only two 
behavioral changes, the rest of the changes are in exposition:
  - Metafactory no longer leaks ReflectiveOperationException
  - Alternate metafactory no longer infers serializability from presence 
of serializable supertype; requires FLAG_SERIALIZABLE.  This makes 
alternate metafacory a pure generalization of the fast-path metafactory.