Package-level declarations

The ReservationStore on MongoDB (documentation/designs/01-programming-model.md §7): one collection per aggregate type, wow_reservation__<context>__<aggregate> (the type is the unit of placement, so its reservations follow its log's store), created with its format record on the type's first use. One document per reserved key, {_id: {t: tenant_id, n: namespace, k: key}, tenant_id, key, owner: {tenant_id, id}, expires_at, holds, version}: the key is the _id (the system tenant is the empty string), its tenant and key also plain fields, the shard key's; the owner is of the collection's type. A null expires_at marks a confirmed reservation, version the owner's fact that last confirmed it; a pending one expires at that epoch millisecond and is held by the attempts in holds, without a version. Every operation finds a reservation by its key, so the collection has no index but the _id's (and the shard key's when sharded). The layout is format FORMAT.