Added option for backend to have a vhost override

[mchandler-plato]

Describe your changes
Allows a backend to specify a different vhost to connect to than the one provided by the client. This allows easy migration of clients to new rabbitmq cluster with a different vhost.

Testing performed
Run setup in production on live rabbitmq cluster with clients using default vhost '/' and redirecting them to other vhosts

Additional context
Prob missing a few things, please let me know what improvements i can make

[adamncasey]

Thank you for the PR!

The workflow makes sense to me. In the past we had discussed holding a vhostname map, e.g.

amqpprox_ctl /tmp/amqpprox BACKEND ADD backend-1 dc hostname 5672
amqpprox_ctl /tmp/amqpprox BACKEND ADD backend-2 dc hostname-2 5672
amqpprox_ctl /tmp/amqpprox MAP BACKEND old-vhost backend-1
amqpprox_ctl /tmp/amqpprox MAP BACKEND new-vhost backend-2

amqpprox_ctl /tmp/amqpprox VHOST REMAP old-vhost new-vhost

This would remove the need to configure backends twice (e.g. if backend-2 also already mapped a few unrelated vhosts).

What do you think?

[adamncasey]

I suggest we use std::optional<std::string> here - mainly for the benefit of indicating that it may not be set.

[adamncasey]

The only property which is encoded/decoded in amqpprox on Open is the vhost so copying the inbound open is a little pointless now, I feel it's neater to keep the Open object immutable and add a constructor which takes a vhost string. I think we could go with something like this, and ditch the d_open property on the Connector

Suggested change

	methods::Open openCopy = d_open;
	openCopy.setVirtualHost(d_sessionState_p->getBackendVirtualHost());
	sendResponse(openCopy, false);
	methods::Open openMethod{d_sessionState_p->getBackendVirtualHost()};
	sendResponse(openMethod, false);

[mchandler-plato]

I wasnt sure what else used d_open so making a copy was 'safer'. Open to doing what ever way makes more sense

[mchandler-plato]

@adamncasey thanks for the review, VHOST remap does sound like a better solution, i take it when we get the open package from the client, we would then look up the mapping and update it?

Will have to wait a few weeks till i get some time to try that change