feat(spooler): Add file backed envelope stacks

[iambriccardo]

This PR implements a new file backend envelope stack, creating a file per own_key project_key combination. The files have a custom format designed for fast access and the data layout is explained in code comments.

The whole idea of this mechanism is that we always append and remove data from the end of the file, meaning no in-memory buffering takes place. When a file is found to be empty, it will be removed from the file system.

In the future, given our file-per-pair model, we could parallelize spools and batch them.