More fixes around variable support:
+ Remove default_dynamic_policy, default_time_zone variables, since defaults
  are now handled as the globally scoped dynamic_policy, time_zone values.
+ Clean up variable support in the parser.  In particular, add support for
  @@global.<foo>, @@session.<foo>, @@<foo>, and so on.
+ Modify the escape keyword infrastructure slightly to handle set global transaction isolation,
  set global names, etc.
+ Clean up use of variable scopes in the parser.
+ Track whether a variable reference is lhs or rhs.  If lhs we generally normalize to the
  global <foo> version.
+ Add info schema table variable_definitions for enquiring about metadata of variables.
+ Add info schema tables session_variables and global_variables.
+ Add support for querying variables with where clauses on the show commands and queries
  against the info schema tables.  For all cases other than the simple select <columns>
  from ... where name like ... we find the variable set, create a temp table, load the variable
  values in to the table and execute the query against the temp table.
+ Add an enumeration for mysql transaction isolation values.  Use the enumeration in place of
  various switch statements.
+ Arrange for the LogicalSchemaQueryEngine to execute queries against the variable tables differently.
+ Remove the hardcoded show variables where .... result set.  Add a ton of variables to known variables
  in order to return the same result set.
+ Add an enumeration of sql modes to SQLMode.  Yank functionality from the variable interface into
  SQLMode, and arrange for the SQL_MODE variable handler to store a SQLMode object.
+ Stub out variables appropriately on the NullConnectionContext, also on the TransientExecutionEngine.
+ Arrange for privilege checking when processing set variable statements.
+ Fix some race conditions in the ServerGlobalVariableStore