sql: replace pgnoRoot with struct space in BtCursor

Instead of passing encoded space id and index id to SQL bindings,
pointers to space and index are saved in cursor and passed implicitly.
Space and index lookups appear during execution of
OP_OpenRead/OP_OpenWrite once. Moreover, having struct space it has
become possible to remove several wrapper-function calls on insertions
and deletions by invoking box_process_rw().

Closes #3122