r7782: fixed an ordering problem with smb requests. I found this when I had "sam...

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