(ITS#4070) back-ldap should resolve referrals itself

[hyc@OpenLDAP.org]

Full_Name: Howard Chu
Version: 2.3.9
OS: 
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (24.126.120.178)
Submitted by: hyc


Watching test039 run, and with some of the discussion about detecting chaining
loops, it's clear that back-ldap should not be relying on libldap to chase
referrals. Instead back-ldap should manage them along with the rest of its pool
of connections. Also the connection record should contain some kind of "last
contacted" timestamp along with server status, so we can reduce the number of
attempts to connect to unavailable servers. (E.g., if server_status == DOWN &&
now - last_contacted < 30, ignore...)

There's still a separate question of how to deal with chaining loops, e.g.
pointing two back-ldap installations at each other. No referrals are involved
there, so the only solution requires adding state to the requests (e.g.
chainingArguments).