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
Useless assignment to variable - `html`. Use just operator `||`.