Opens a session for a block of nbFrag fragments of fragSize bytes.
maxLost is the most uncoded fragments to be able to solve for, which decides how
much working memory the session takes.
The block, as far as it has been put back together, padding and all.
Whether the whole block is there.
How many uncoded fragments are still missing.
How many fragments arrived, coded, uncoded and repeated.
A fragmentation session being put back together, TS004-2.0.0 appendix A.2.
The uncoded fragments go straight into the block. A coded fragment is reduced against everything already known and kept only if it says something new, so the working memory is sized by the losses rather than by the block.