Previously, Zeek treated the receipt of `AuthenticationOk` as a
successful login. However, according to the PostgreSQL
Frontend/Backend Protocol, the startup phase is not complete until
the server sends `ReadyForQuery`. It is still possible for the server
to emit an `ErrorResponse` (e.g. ERRCODE_INVALID_AUTHORIZATION_SPECIFICATION)
after `AuthenticationOk` but before `ReadyForQuery`.
This change updates the PostgreSQL analyzer to defer reporting login
success until `ReadyForQuery` is observed. This prevents false
positives in cases where authentication succeeds but session startup
fails.
