nvx: list a template store's leases in one pass #1005

nvx::host::template::Store::leases(generation) scans the store's lease directory and reads every lease record to return those of one generation. A caller that lists the leases of each of G generations therefore reads each of L records G times.

A tool that reports every generation of a store with its leases, such as the runtime adapter's list and inspect, needs them all at once.

Add a public listing of all of a store's leases in one pass, for example Store::all_leases(), grouped by generation, with the same consistency as leases. Then use it in the runtime adapter's generations::entries.

Found in #962 (a review thread on the runtime adapter's list). Related to #833.