Optimize consecutive message field assignments

[treiher]

Consecutive assignments to fields of the same message should be realizable with fewer checks:

• Check Available_Space once
• Check Valid_Next once

The assignments could be collected into a new object ("delta message aggregate"). It has to be checked that the set fields are a valid sub-path of the message. For the calculation of the required size, Message.size must consider message sub-paths.

Note: Field_Condition has to be checked on certain fields in case a condition could be violated. Field_Condition is not yet checked and adding of such checks is not part of this ticket, but will be done in #1121.