Added incremental build output

- We pass converter state to client side
- Client side when next time requests build output sends last received converter state
- Converter state is restored and generation is resumed from that place
- All span's are properly restored
- Currently there is simple check if build changed: offset present in state is within currently saved build output. This can be enhanced to introduce some simple checksum: CRC16 or CRC32 should be sufficient.

It's made to greatly reduce network traffic and increase performance of html generation.

THIS IS PROOF OF CONCEPT HOW IT CAN BE DONE
Redundant `self` detected.