reduce IP space fragmentation

[rade]

The IPAM IP space can become very fragmented. In extremis we end up with one token per IP address. Which makes IPAM expensive for both CPU and bandwidth.
#1159 brought up one scenario that might introduce extreme fragmentation.

[tomwilkie]

I can't see a way of guaranteeing we don't get fragmented, which therefore means to reduce fragmentation you need to remove entries in the ring - which means we need to reintroduce tombstones.

Some of the original ideas were to periodically "give up" any empty range you might own to your previous owner by tombstoning your entry. Another was to periodically tombstone any entries you own which split spaces you own.