sql: return result-set type via IProto

Lets evaluate an expression type during processing of expression's AST
and code generation. It allows to calculate resulting columns data types
and export them as IProto meta alongside with columns' names.
Also, correct types are also returned for binding parameters as well.

Note that NULL literal has type "BOOLEAN". It was made on purpose -
different DBs interpret NULL's type in different ways: some of them
use INT; others - VARCHAR; still others - UNKNOWN. We've decided that
NULL is rather of type "BOOLEAN", since NULL is kind if subset of
"BOOLEAN" values: any comparison with NULL results in neither TRUE nor
FALSE, but in NULL.

Part of #2620