client.Do() is not safe for concurrent use

[gnuletik]

During automated tests, that runs concurrent rdap queries, I've seen the following panic:

github.com/openrdap/rdap/bootstrap.(*Client).Lookup(0xc00118e690, 0xc000e80960)
	/home/actions-runner/go/pkg/mod/github.com/openrdap/rdap@v0.9.1/bootstrap/client.go:340 +0x4d4
github.com/openrdap/rdap.(*Client).Do(0xc000427bf0, 0xc000e923c0)
	/home/actions-runner/go/pkg/mod/github.com/openrdap/rdap@v0.9.1/client.go:150 +0x4c5

I was able to reproduce this issue locally while running

$ go test -race
==================
WARNING: DATA RACE
Read at 0x00c00047acc0 by goroutine 33:
  github.com/openrdap/rdap.(*Client).Do()
...

Previous write at 0x00c00047acc0 by goroutine 40:
  github.com/openrdap/rdap.(*Client).Do()
      /home/actions-runner/go/pkg/mod/github.com/openrdap/rdap@v0.9.1/client.go:96 +0xe4

Would it be possible to make the client.Do function safe for concurrent use?
If not, would it be possible to show it in the documentation?

Thanks!