This commit ensures that Alloc.AllocatedResources is properly populated
when read from persistence stores (namely Raft and client state store).
The alloc struct may have been written previously by an arbitrary old
version that may only populate Alloc.TaskResources.

Now that alloc.Canonicalize() is called in all alloc sources in the
client (i.e. on state restore and RPC fetching), we no longer need to
check alloc.TaskResources.

alloc.AllocatedResources is always non-nil through alloc runner.
Though, early on, we check for alloc validity, so NewTaskRunner and
TaskEnv must still check.  `TestClient_AddAllocError` test validates
that behavior.

alloc.Job may be stale as well and need to migrate it.  It does cost
extra cycles but should be negligible.

There is a case for always canonicalizing alloc.Job field when
canonicalizing the alloc.  I'm less certain of implications though, and
the job canonicalize hasn't changed for a long time.

Here, we special case client restore from database as it's probably the
most relevant part.  When receiving an alloc from RPC, the data should
be fresh enough.