options_from_collection_for_select helper takes html attributes

as the last element of the array
switch (and lazily convert) ivar names to 3.x
let EXPLAIN use a thread locals registry [John J. Wang & Xavier Noria]

Closes #10198.
Merge pull request #10173 from nashby/element-attributes

options_from_collection_for_select helper takes html attributes
Merge pull request #10239 from rubys/cache_entry_conversion_problems

switch (and lazily convert) ivar names to 3.x
more descriptive NameError's messages for class attributes
Merge pull request #10249 from shime/descriptive_error_messages

more descriptive error messages for class attributes