LDAPSERVER was documented as a space separated list, but the whole
string went into a single ldap3.Server. Logins worked (python-ldap
accepts lists), while registration and every write would have failed
with more than one URI.
settings.LDAP_SERVERS is now the split list, used through one ldap3
ServerPool per process: requests go to the first reachable server, a
server that is down is skipped for 60s, connections time out after 5s,
and with all servers down the pool gives up after two passes instead
of retrying forever.