Add per-client close hook for deterministic cleanup of connection-scoped resources

[tyr1k]

Hi,

ldapserver already supports per-connection client data via SetData / GetData, and Unbind is handled internally to close the client connection.

For LDAP proxy use-cases it would be very helpful to have a public per-client close hook, so resources attached to that client session can be released deterministically when the frontend LDAP connection ends.

Typical example:

• on Bind, the proxy opens a backend LDAP connection
• stores it in Client.SetData(...)
• on frontend disconnect / unbind, that backend connection should be closed immediately

Today this has to be implemented indirectly (for example via idle reapers / timeouts), because there is no explicit callback when the client session is closed.

[tyr1k]

#47