Docs for SERVER-34260: Ability to reuse a single TCP connection from mongod to the LDAP server

Description

Description:

This adds a connection pool to manage LDAP connections used for LDAP authn/authz. In 4.2 it is on by default if mongod is linked to the thread-safe version of openldap on Linux, and always enabled on Windows. There is also a set parameter to disable it.

Engineering Ticket Description:

When enabling LDAP in the typical setting, mongod process is using the three separate TCP connections to the LDAP server(s) for every db.auth() command:
1. One (or more) connections for the sections in the security.ldap.userToDNMapping option with the ldapQuery predicates
2. A single connection to authenticate the user by using the bind operation
3. A single connection to obtain the list of groups for authorization.

I am wondering if we could create a single TCP connection during the step 1 or 2 and reuse it for the next steps without reestablishing connection every time for the particular thread (==session) in the server?

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)