For those SP instructions that need to get access to ia LEX object
on execution, added storing of their original sql expressions inside
classes derived from the class sp_lex_instr.

A stored sql expression is returned by the abstract method
  sp_lex_instr::get_expr_query
redefined in derived classes.

Since an expression constituting a SP instruction can be invalid
SQL statement in general case (not parseable statement), the virtual
method sp_lex_instr::get_query() is introduced to return a valid string
for a statement that corresponds to the given instruction.

Additionally, introduced the rule remember_start_opt in the grammar.
The new rule intended to get correct position of a current
token taking into attention the fact whether lookahead was done or not.