Run all LDAP operations in thread pool

We've found that LDAP operations can hang if establishing a new LDAP connection needs to perform side-channel operations - such as GSSAPI auth for the LDAP lookup user - that don't respect the LDAP timeout values. Since we introduced a thread pool for doing LDAP connection pooling, we should just run all LDAP operations inside that thread pool so we can abandon long-running operations and return a quick and reliable error to the user on failure rather than hanging for forever.