core: improve fast-path multifragment handling

* make sure fast-path packages are not fragmented if no
  multifragment support was announced
* handle special server side case where the multifragment size
  received from the client is smaller than one maximum fast-path
  PDU size
Merge pull request #1989 from bmiklautz/feat/fast-path

core: improve fast-path multifragment handling