Implement memory quota tracking

The branch is on top of !2483 (merged) and !2459 (merged). These are Approved but at the time of writing they have not merged. You can start your review with the empty commit titled BASE, which is on top of the merge of those two base branches.

The branch structure is as follows:

• Provide some types to distinguish accounts at various layers
• Error handling (new error variants, and making a few functions fallible)
• Make accounts of the right levels and plumb them through to where they're needed
• Change the types of the MPSC queues to the memquota ones

After this, all that's left to be done is some polish: documentation, cargo feature fettling, adding to the example config, etc.

Part of #351.