cnderrauber/samplebuilder

This is a drop-in replacement for Pion's samplebuilder that is faster and
has slightly different behaviour. It is used by Galene https://galene.org.

Differences from Pion:

• the Pop family make slightly different decisions about when to
  release a packet that is blocked by an incomplete packet, with the
  goal to minimise latency in the lossless case while avoiding spurious
  packet drops (it's a tricky decision, and Pion itself behaves
  differently depending on the exact version);

• the ForcePopWithTimestamp function can be used to flush any
  packets remaining at the end of the stream;

• we're about 30% faster.