Index hosts by IP address in the host store

The HostService allows looking up hosts by IP address, but the current implementation requires linearly searching through the consistent map to find hosts with a given IP address.

This is a performance bottleneck, and performance degraded when the ECMap was replaced with a consistent map.

We can improve this by keeping an in-memory index of IP address -> Host in the HostStore. The index would be updated by events from the consistent map.