Custom cursors support

There is a problem with cursor based pagination on requests by secondary indexes. Consider a simple key-value schema:

The call crud.select('kv', {{'=', 'tag', 'tagValue'}}, {first = 10, after = ...}) makes a fullscan of secondary index from starting point (tagValue) to tuple equaling cursor. Under heavy load it leads to CPU consumption and cluster performance degradation.

To solve this problem, we have to create a special unique index paging with two fields {tag, key} and implement pagination manually, by executing such calls like

local rs = crud.select('kv', {{'==', 'paging', {'tagValue', box.NULL}}}, {first = 10})

...

local last_key = rs.ros[#rs.rows][0]
rs = crud.select('kv', {{'>', 'paging', {'tagValue', last_key}}}, {first = 10})
...

It would be nice to introduce some feature like passing name of paging index and using key from this index as cursors