* topology: keep per-node volume state with the location it describes
The read-only and oversized indexes were maps from volume id to a list of the
nodes reporting that state -- the same key space the lookup index already
holds, kept a second and third time. Nothing ever asked which nodes; both are
only ever asked whether any node does.
So the state rides on the location list as a bit per entry, and the two indexes
go. Removing a location shifts the bits with it, and a node replacing another
at the same address inherits its slot, since that is what happens to the
location too.
800k volumes, 90% read-only: readonly index 40.0MB -> 0, lookup index
42.5MB -> 48.6MB for the bits, 33.9MB net.
* topology: rebuild the location flags when stale entries are dropped
Refresh rebuilds the location list, so leaving the flags alone left bits
describing whoever moved into the dropped entries' place.
* topology: assert the refreshed flag survived, not just that it moved
Clearing the mask rather than rebuilding it would have passed: the check that
the flag lands on the right location is done by clearing it, which an already
empty mask satisfies.
