destCh was being written to by one goroutine and closed by another
goroutine. This panic occurred in Travis:
```
=== FAIL: drivers/docker TestDockerCoordinator_ConcurrentPulls (117.66s)
=== PAUSE TestDockerCoordinator_ConcurrentPulls
=== CONT TestDockerCoordinator_ConcurrentPulls
panic: send on closed channel
goroutine 5358 [running]:
github.com/hashicorp/nomad/drivers/docker.dockerStatsCollector(0xc0003a4a20, 0xc0003a49c0, 0x3b9aca00)
/home/travis/gopath/src/github.com/hashicorp/nomad/drivers/docker/stats.go:108 +0x167
created by
github.com/hashicorp/nomad/drivers/docker.TestDriver_DockerStatsCollector
/home/travis/gopath/src/github.com/hashicorp/nomad/drivers/docker/stats_test.go:33 +0x1ab
```
The 2 ways to fix this kind of error are to either (1) add extra
coordination around multiple goroutines writing to a chan or (2) make it
so only one goroutines writes to a chan.
I implemented (2) first as it's simpler, but @notnoop pointed out since
the same destCh in reused in the stats loop there's now a double close
panic possible!
So this implements (1) by adding a *usageSender struct for handling
concurrent senders and closing.
Track current memory usage, `memory.usage_in_bytes`, in addition to
`memory.max_memory_usage_in_bytes` and friends. This number is closer
what Docker reports.
Related to https://github.com/hashicorp/nomad/issues/5165 .
