Miscellaneous fixes and cleanup

[ebatsell]

Fixes a number of small things including:

• Potential for self invocation of CopyGossipContactInfo to overwrite real values
• Reading data offsets in CopyGossipContactInfo rather than hardcoding assumed offsets
• Potential epoch overflow for cast epoch
• Comments around how specific structs and instructions are intended to work
• Shifting epoch_start_timestamp to avoid overwriting existing bytes and avoid trimming data type