otherwise we can re-insert the entry into the map after a parallel
allocateAgain() has cleared the flag. And it /that/ happens in
parallel with a free() then the data-structure can get out of sync.
It was possible to free an address between deciding to re-test it
and actually re-testing it. So introduced a flag 'block' to prevent
an address from being freed during the re-test.
Using the Ring CRDT and Weave gossip, provides an interface
for the weave script to allocate IP addresses in `weave run`,
`weave start` and `weave expose`.
See docs/ipam.md for implementation details, and site/features.md
for user-visible changes.
Bryan Boreham