Fix Memory Leak in ExportBgen

[johnc1231]

CHANGELOG: Fix memory leak in ExportBgen

If you look a dozen or so lines under the boundary I added, you'll see a :

        it.foreach { ptr =>
          val (b, d) = bpw.emitVariant(ptr)
          out.write(b)
          dropped += d
        }

So that thing is stepping through the iterator without freeing. The boundary is to do a clear after each read from the it.

[johnc1231]

Trying to fix: https://discuss.hail.is/t/limit-memory-usage/1468/3

[patrick-schultz]

Good catch!