Rename PostAllocate to Allocate in Multi Cluster Allocation Service

[markmandel]

This came up in #1314 (comment)

In this service:
https://github.com/googleforgames/agones/blob/master/proto/allocation/v1alpha1/allocation.proto#L22

We agreed that Allocate would be a nicer name to use.

[markmandel]

/cc @pooneh-m for visibility

[markmandel]

Thought I had - we could retain and deprecate PostAllocate for a release or two, and keep it alongside the new Allocate Service, to give people a smoother transition before removing it.

[pooneh-m]

The API is marked as alpha, which means there is no backward compatibility guarantee. This rpc method name change will not be the only breaking change and either we should support every breaking change or this will not be an exception. Because for V1 version the backward compatibility between alpha and v1 will not be carried over, I recommend breaking the API now and support backward compatibility on a stable version release.

[markmandel]

That's also fair. I would put it into the "best effort" bucket - if it's easy to do, awesome, if not, I don't have a strong opinion to maintain backward compatibility.

[pooneh-m]

It is easy to do, but IMHO it is unnecessary.