BUG/MINOR: backend: do not overwrite srv dst address on reuse

BUG/MINOR: backend: do not overwrite srv dst address on reuse

Previously, destination address of backend connection was systematically
always reassigned. However, this step is unnecessary on connection
reuse. Indeed, reuse should only be conducted with connection using the
same destination address matching the stream requirements.

This patch removes this unnecessary assignment. It is now only performed
when reuse cannot be conducted and a new connection is instantiated.

Functionnally speaking, this patch should not change anything in theory,
as reuse is performed in conformance with the destination address.
However, it appears that it was not always properly enforced. The
systematic assignment of the destination address hides these issues, so
it is now remove. The identified bogus cases will then be fixed in the
following patches.would

This should be backported up to all stable versions.