Implement box.cfg { read_only = true }

bool is_ro is now controlled by box.cfg{} and is not used by
register/unregister logic in _cluster table.

[documentation][skip ci]

Conforming changes to gh-1334, change wal size to 5 millions, arena size to 0.5 GB and give ability for user 'guest' to read/write into 'example' space.

Automatically close the cursor after a last read key.

Re-implement SophiaIndex/iterator with an api which does not invoke
sp_set* code on every scanned row.

See #1349