PSA: redesign slot management to keep track of what's using a key

[gilles-peskine-arm]

The PSA slot management code in Mbed TLS doesn't keep track of what a key may currently be used for: a volatile handle, an open persistent handle, an operation. Redesign it so that it's possible, given a key slot index, to know what the slot is currently used for.

The goal of this task is to have a design. Code is optional. The design must be suitable to:

• invalidate an operation in progress when a key is destroyed explicitly;
• destroy a key when it's no longer in use and not explicitly opened.

A possible solution is a reference count in the slot.