Adjust the transport seam to ensure that incoming message modifications can't persist between immediate retry attempts

[SzymonPobiega]

If the immediate retries are implemented in-memory within the transport the current transport seam makes it possible to modify the incoming message so that the effects of the modification are visible in the subsequent attempts:

• Headers dictionary can modifications are retained
• Substituting whole message body is handled properly (changes are reverted) but substituting individual bytes in the body byte array is visible in the subsequent reties

[dbelcham]

This work was prioritized by KtLO but couldn't be worked on because it is a breaking change. KtLO would like to have this considered and prioritized for the next major of Core.

[timbussmann]

(adding this to the v8 milestone for now as KtLO expressed willingness to make this a priority once the v8 window opens up)