tests: add concurrent UDS writing test
tests: add concurrent UDS writing test

This commit will make the CI fail, as this demonstrates utilizing the
UDS writer in a concurrent program is currently unsafe.

This is triggered because the current UDS class changes the underlying
`conn` pointer on errors unsafely.

This would show up as:
```runtime error: invalid memory address or nil pointer dereference```

Running the `TestClientUDSConcurrent` test with the race detector shows
this as well.
```
==================
WARNING: DATA RACE
Read at 0x00c420126000 by goroutine 16:
  [...]
  github.com/DataDog/datadog-go/statsd.(*udsWriter).Write()
      /Users/leo/datadog/go/src/github.com/DataDog/datadog-go/statsd/uds.go:52 +0x14e
  [...]

Previous write at 0x00c420126000 by goroutine 13:
  [...]
  github.com/DataDog/datadog-go/statsd.(*udsWriter).Write()
      /Users/leo/datadog/go/src/github.com/DataDog/datadog-go/statsd/uds.go:46 +0x328
  [...]
```

DataDog/datadog-go