If one thread calls `Flush()` on a gatedwriter while another thread attempts to
`Write()` new data to it, strange things will happen.
The test I wrote shows that at the very least you can write _while_ flushing,
and the call to `Write()` will happen during the internal writes of the
buffered data, which is maybe not what is expected. (i.e. the `Write()`'d data
will be inserted somewhere in the middle of the data being `Flush()'d`)
It's also the case that, because `Write()` only has a read lock, if you had
multiple threads trying to write ("read") at the same time you might have data
loss because the `w.buf` that was read would not necessarily be up-to-date by
the time `p2` was appended to it and it was re-assigned to `w.buf`. You can see
this if you run the new gatedwriter tests with `-race` against the old implementation:
```
WARNING: DATA RACE
Read at 0x00c0000c0420 by goroutine 11:
runtime.growslice()
/usr/lib/go/src/runtime/slice.go:125 +0x0
github.com/hashicorp/nomad/helper/gated-writer.(*Writer).Write()
/home/hicks/workspace/nomad/helper/gated-writer/writer.go:41 +0x2b6
github.com/hashicorp/nomad/helper/gated-writer.TestWriter_WithMultipleWriters.func1()
/home/hicks/workspace/nomad/helper/gated-writer/writer_test.go:90 +0xea
```
This race condition is fixed in this change.
