pgsql: Remove unnecessary unbind in LDAP search+bind mode

Remove unnecessary unbind in LDAP search+bind mode

Comments in src/backend/libpq/auth.c say: (after successfully finding
the final DN to check the user-supplied password against)

/* Unbind and disconnect from the LDAP server */

and later

/*
* Need to re-initialize the LDAP connection, so that we can bind to
* it with a different username.
*/

But the protocol actually permits multiple subsequent authentications
("binds") over a single connection.

So, it seems like the whole connection re-initialization thing was
just a confusion and can be safely removed, thus saving quite a few
network round-trips, especially for the case of ldaps/starttls.

Author: Anatoly Zaretsky <anatoly(dot)zaretsky(at)gmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/CALbq6kmJ-1+58df4B51ctPfTOSyPbY8Qi2=ct8oR=i4TamkUoQ(at)mail(dot)gmail(dot)com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f8d03ea727e48c894174c4e5e6ebafc3a17ab3f7

Modified Files
--------------
doc/src/sgml/client-auth.sgml | 6 +++---
src/backend/libpq/auth.c | 25 -------------------------
2 files changed, 3 insertions(+), 28 deletions(-)