PE-1587, PE-1589, PE-1590 - Added global system variables.
PE-1591 - Added unit test.
Lookup EnumVariableValues by toString() rather than name()
implementation.
Add result set metadata fixes, specifically proper computation of column aliases.
Instrument projection parsing to find derived column boundaries and store it in source
location information.   During projection metadata computation, arrange to use such
source location in building derived column aliases.

After finally fixing this (after at least a year), I decided to uncategorically turn
on metadata comparison for mirror tests, which yielded several more issues:
+ The information_schema was persisted in the wrong case.  Fix this, and arrange
  for a case insensitive lookup.
+ At least part of the info schema presented the wrong column types.  Fix these
  as I found them.
+ Add another infoschema upgrade.

Still todo:
+ Figure out what to do about ignoreMD in ComparisonOptions.