NewAllocWatcher creates a PrevAllocWatcher appropriate for whether this alloc's previous allocation was local or remote.

NoopPrevAlloc does not block or migrate on a previous allocation and never returns an error.

AllocRunnerMeta provides metadata about an AllocRunner such as its alloc and alloc dir.

PrevAllocMigrator allows AllocRunners to migrate a previous allocation whether or not the previous allocation is local or remote.

PrevAllocWatcher allows AllocRunners to wait for a previous allocation to terminate whether or not the previous allocation is local or remote.

RPCer is the interface needed by a prevAllocWatcher to make RPC calls.