iproto: add pagination options to iproto

The patch adds new iproto keys needed for pagination and extends
tx_process_select, used by IPROTO. IPROTO and NETBOX_IPROTO versions
are updated, iproto feature pagination is introduced, new keys are:

0x2e - IPROTO_AFTER_POSITION - start iteration after passed
iterator position. It has type MP_STR.

0x2f - IPROTO_AFTER_TUPLE - start iteration after passed tuple.
It has type MP_ARRAY.

0x1f - IPROTO_FETCH_POSITION - send position of last fetched tuple
in response. It has type MP_BOOL.

0x35 - IPROTO_POSITION - iterator position, sent in response if
IPROTO_FETCH_POSITION is true. It has type MP_STR.

Part of #7637

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