Apply search authorization to persistent search notifications

[coheigea]

Apply search authorization to persistent search notifications
Entries sent to a persistent search client were taken straight from the
operation context, bypassing the authentication and ACI checks applied
to a normal search, as well as the userPassword hiding. With changesOnly
set, no check at all was done on the client before registering it.

• PersistentSearchListener now reads every changed entry again through
  the client's own session, with a base level search, and only sends it
  if it is visible, with the attributes the client is allowed to read.
  userPassword is removed when the server hides passwords.
• As a deleted entry can't be read anymore, DirectoryListener gets a
  beforeEntryDeleted() callback, called by the EventInterceptor before
  the entry is deleted.
• A changesOnly persistent search now does a base level search through
  the client's session before the listener is registered, so that an
  unauthenticated client is rejected when anonymous access is disabled.
• The EventInterceptor executor queue is now unbounded: the listeners
  wait for the end of the operation which notifies them, and a rejected
  notification made that operation fail.