v4: Async LDAP connection fixes (#4240)

v4: Async LDAP connection fixes (#4240)

* Improve log message for successful LDAP async bind

* typo

* net_timeout is still used

* ldap_get_option can return LDAP_SUCCESS before the fd is known

* If _ldap_bind_io_write was called without an fd look it up now

* Store the connection fd for setting trunk events

* Signal the connection once the bind has succeeded

* Setting LDAP_OPT_NETWORK_TIMEOUT actually stops async calls

* log_prefix should be const

* Associate connection state handle with ldap connection

* Add error message if LDAP connection does not allocate

* Explain use of net_timeout

* Explain behavior of ldap_get_option

* Explain why we may call _ldap_bind_io_write() without a writable fd

Co-authored-by: Arran Cudbard-Bell <a.cudbardb@freeradius.org>