r7782: fixed an ordering problem with smb requests. I found this when I had "sam database"
set to the internal ldap server over loopback. The following happened:
- DCERPC_AUTH3 request
- auth requests calls ldb
- ldb calls ldap
- ldap calls our internal ldap server, triggering events
- samrConnect from client
- connect refused
- SMBclose from client
- causes dcerpc_pipe to be destroyed
- AUTH3 continues
- dies on freed pipe
I chose this solution as it provides a guarantee that backends only have to think about
async issues when they mark a request async. When they don't, this code guarantees that
a second request won't happen on the same connection while processing the first one