clarify CHANGES for bootstraping w/ plugin=mysql_native_user
bring CHANGES up-to-date with all merged branches
add documentation for metadata, version sandbox.sh actions
make sandbox.sh restart behave similarly to start

- command line options are passed on to start so that
  ./sandbox.sh restart --init-file=/tmp/reset_root.sql behaves
  similarly to ./sandbox.sh start --init-file=/tmp/reset_root.sql
note ./sandbox.sh restart [options] change