Defines the BigPipe placeholder strategy, to send HTML in chunks.
First: the BigPipe placeholder strategy only activates if the current request is associated with a session. Without a session, it is assumed this response is not actually dynamic: if none of the placeholders show session-dependent information, then none of the placeholders are uncacheable or poorly cacheable, which means the Page Cache (for anonymous users) can deal with it. In other words: BigPipe works for all authenticated use