sql: use region_alloc/join for temporary sql iproto tuples

obuf_alloc can not be used as a temporary buffer per sql row column,
because each column can be allocated in different slabs. It makes
impossible to allocate a port tuple from several intervals:
[col1_begin, col1_end], [col2_begin, col2_end], ...

Region has region_join method, which can realloc memory blocks from
different slabs in monolite memory area - [begin, end].
It allows to use tuple_alloc for port tuples.

Closes #2602