Add a way to differentiate the listen addresses for each node of a cluster

Every Gateway node in a cluster share the same configuration and the same database. They have the same definition of servers, including the listenAddress.

This poses a problem, because they cannot all listen on the same IP address.

Proposed solution

Add a table in the database to override settings for a given node of the cluster.

Each node must be given a identifier unique to the cluster. The table could have the following structure:

• Cluster id = Gateway name
• Node id
• Key
• Value
• PK: (clusterid, node id, key)

The node identifier will be given as a command line argument when the server is launched, with support for an env var and a fallback on the hostname.

When a listener is started, the table is searched for an override for the node with the key listenAddress::OLDADDRESS and the value NEWADDRESS where:

• OLDADDRESS is the address to replace (typically the address of the load balancer)
• NEWADDRESS is the address to use

Key examples:

• listenAddress::127.0.0.1
• listenAddress::lb.fully.qualified.name
• listenAddress::[67b4::92a3]

If none is found, the server's listenAddress is used, making it possible to use other external mechanism (NAT, /etc/hosts, etc.)