MINOR: cache/filters: Initialize the cache filter when stream is created

MINOR: cache/filters: Initialize the cache filter when stream is created

Since the HTX mode is the only mode to process HTTP messages, the stream is
created for a uniq transaction. The keep-alive is handled at the mux level. So,
the cache filter can be initialized when the stream is created and released with
the stream. Concretly, .channel_start_analyze and .channel_end_analyze callback
functions are replaced by .attach and .detach ones.

With this change, it is no longer necessary to call FLT_START_FE/BE and FLT_END
analysers for the cache filter.