sql: use mem_mp_type() in sql_value_type()

sql_value_type() and mem_mp_type() do the same thing: return messagePack
type corresponding to value stored in memory cell. However,
sql_value_type() operates on opaque API wrapper - sql_value*. To avoid
duplicating code let's invoke mem_mp_type() in sql_value_type().
At once, let's account that mp_type now can be not only _BIN, but also
_ARRAY and _MAP - this fact will be used when we introduce arrays and
maps in SQL.