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Add synchronization to prevent post-melt collects

This fixes a race condition in which snowflakes.End() is called while
snowflakes.Collect() is in progress resulting in a write to a closed
channel. We now wait for all in-progress collections to finish and add
an extra check before proceeding with a collection.
Status Job ID Name Coverage
  Test
canceled #4212
android

canceled #4215
debian-testing

canceled #4213
go-1.13

canceled #4214
go-1.14