This is the prerequisite patch introducing the class sp_lex_instr
that encapsulates access to an instance of the class sp_lex_keeper.
Every SP instruction that need to get access to a LEX object on its
processing should inherit this class and implement two abstract methods:
  is_invalid(),
  invalidate().

These methods will be used in subsequent patches to implement recompilation of
SP instructions on failure.

Currently, the following instructions are derived from the class sp_lex_instr:
  sp_instr_stmt,
  sp_instr_set,
  sp_instr_set_trigger_field,
  sp_instr_jump_if_not,
  sp_instr_freturn,
  sp_instr_cpush,
  sp_instr_cursor_copy_struct,
  sp_instr_set_case_expr

Additionally, this patch converts the class sp_instr_opt_meta
to a base abstract class (that is, not inherited from the class
sp_instr). Since this class originally was designed to provide a way
for opimizer to update a destination address for jump SP-instructions,
the only useful method at the interface of this class is set_destination
and therefore inheritance from the class sp_instr is meaningless.

Every jump SP instruction now must be inherited directly from
the class sp_instr_opt_meta and additionally from either the class
sp_lex_instr or sp_instr depending on whether this SP instruction
need to get access to a LEX object or not.

Moreover, the class sp_cursor doesn't own a data member of
the class sp_lex_keeper any more. Instead, the virtual method
get_lex_keeper() has been added to the class sp_cursor() that
returns nullptr and this method is overridden in the derived class
sp_instr_cpush to provide a pointer to a real instance of
the class sp_lex_keeper. Doing this way we exclude duplication
of a data member of the type sp_lex_keeper at the class sp_instr_cpush
since it is derived both from sp_lex_instr and sp_cursor, and sp_lex_instr
already encapsulates a data member of the class sp_lex_keeper.