sql: rework 'DROP INDEX' and 'DROP TABLE' handling

As a part of SQL data dictionary integration, 'DROP INDEX' and
'DROP TABLE' statements proccessing has been refactored in order
to avoid using SQL specific internal structures.
However, triggers still aren't transfered to server, so to drop them
it is required to lookup SQL table and emit apporpriate opcodes.
Also, added comments and fixed codestyle for functions processing
'DROP' routine.

Part of #3222.