Extend protodump.cc to be able to load another dump and continue from there.

[iphydf]

This would allow us to minimize the initial output from protodump (270KB) to something more reasonable (25KB) and then generate the remaining protocol dump after replaying the 25KB dump. The suffix of the 270KB dump is incompatible with the minimized version, so we need to first get into the Tox state generated by the minimized dump and then output the "send message" trace suffix.