Currently the circulation manager can only have a single storage integration, which is used site-wide.

IIRC we did it this way purely to make it easier to write the integration configuration code.

There are now use cases for associating different storage integrations with different collections. In the medium term there will probably be multiple different types of storage integrations, such as LCP servers.

So let's start a little cleanup by making it possible for the circulation manager to have multiple storage integrations.

The most challenging part of this will probably be replacing all the bits of code where we assume there is a single globally-accessible storage integration that we can use however we want.