Pull changes from 0.12.0 release branch into master

Scrub changes and pull forward what's needed. For certain we need:

1. CHANGES file
2. In lib/dart, "git mv LICENSE_HEADER LICENSE". This fixes a dart pub warning.
3. In release 0.12.0 the thrift version in perl is "v0.12.0_0" instead of "v0.12.0".