BUG/MINOR: stream: Fix custom timeouts initialization when setting backend

BUG/MINOR: stream: Fix custom timeouts initialization when setting backend

Timeouts may be overwritten via the action "set-timeout". Client timeout is
supported on frontends, connect/queue/tunnel/server timeouts are supported
on backends and tarpit timeout is supported on both.

However, there is an issue with the listener. In that case, the action is
always executed in the frontend context and the backend-side timeouts are
then reset, when the backend is selected. If it is another backend, it is
logical. But most of time, the backend is unchanged. And in that case, these
timeouts must be preserved.

Documentation of set-timeout action was improved.

This patch should be backported to all supported versions. Be carefull, all
timeouts are not supported in 3.3 and lower (3.3->3.0: server/tunnel/client
- 2.8->2.6: server/tunnel)