share LDAP server pools between authorization plugins

[vladak]

Another post-mortem induced issue: Whenever a LDAP lookup times out (say with 3 second timeout) I can see the overall request taking disproportionate time (tens/hundreds of seconds), likely depending on the number of projects/groups in the setup (for 40ish groups and 3 seconds timer there was a latency of 117 seconds which is almost the product. This is because the authorization stack is based almost solely on project groups). This happens even with a failover in place.