sql: rework SQL_FUNC_COUNT flag semantics

Tarantool's SQL engine generates a different VDBE bytecode
for ..COUNT(*).. and ..COUNT(fieldname).. operations:
the first one produces a lightweight OP_Count operation that uses
native mechanism to report the count of record in index while
the second one pessimistically opens a space read iterator and
uses Count aggregate function.

A helper routine is_simple_count decides whether such
optimisation is correct. It used to use SQL_FUNC_COUNT flag to
mark a dummy (non-functional) function entry with 0 arguments.
This patch changes SQL_FUNC_COUNT semantics: now it is a marker
of any COUNT function, while is_simple_count relies on count
of arguments to distinguish aggregate and non-aggregate
functions.

Needed for #2200, #4113, #2233