BUG/MINOR: stream: Fix custom max-retries initialization when setting backend

BUG/MINOR: stream: Fix custom max-retries initialization when setting backend

It is possible to overwrite the configured max-retries value with the
"set-retries" action. However there is an issue with the listeners. When the
backend is set, this value is reset with the backend value. However, when
the backend is unchanged, the custom value must be preserved.

To fix the issue, when the stream is created, we set the max-retries to the
listener value. It remains 0 for pure-frontend. And the backend value is
used only if it is not the same proxy.

The documentation of set-retries action was improved.

This patch should be backported as far as 3.2.