In previous version of the format Object message included two header fields:
SystemHeader and list of ExtendedHeader. Due to the fact that all headers
were not encapsulated in a single message, some messages were forced to use
Object to convey the full header of the object (e.g. object.HeadResponse).
To resolve this semantic inconsistency, this commit defines a Header
message, consisting of a system and an extended portion. As a result, the
Object now consists of two fields - Header and Payload.
Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>