Potential race condition in client handling

[kula]

vault-plugin-secrets-backblazeb2/plugin/b2_client.go

Lines 37 to 38 in 3e8eb2c

	b.clientMutex.RUnlock()
	return b.client, nil

I suspect there's a race condition here:

1. We don't defer the release because there's two places we may release it, depending on if there's a current client or not
2. We have to release the RLock before we call newB2Client because it needs to get a Lock

Check the handling of this --- I removing line 37 and doing a defer b.clientMutex.RUnlock() right before 36 would work.