Update function now adds intermediate nullable fields (if they needed)

[mRrvz]

Fixed not fining field in tuple on crud.update if there are is_nullable fields in front of it that were added when the schema was changed. Closes #113

[olegrok]

Thanks for you patch consider several comments below.

I've put quite a lot comments, so I need to clarify some expectations from this patch.

• Updates should be optimistic - we shouldn't get tuple before we understand that some field is missing: error has code box.error.NO_SUCH_FIELD_NO or box.error.NO_SUCH_FIELD_NAME
• This logic should be performed on storage-side
• We should try to do it only for top-level fields (don't think about jsonpath updates)
• Tarantool allows to add fields after fields that declared in space format, but I think it's not general purpose case IMO - so we need to do something only if field name is in space format (but for some reasons some fields is missed before this field) or field number <= space format length.

[olegrok]

Thanks for your fixes. Consider a couple of comments below.

[dokshina]

Please, consider my comments and update a Changelog.

[olegrok]

Thanks for your changes and fixes. Sorry for some controversial comments before.

Currently I basically agree with your patch and with your approach. I put some comments about some details. Please ask @dokshina for the second review.

[dokshina]

Nice work!