Reimplement discontiguous index mode

[jadamcrain]

The API allows for non-valid configurations. It would be way better to pass in a set of maps (i.e. map<index, point-info>). We can then provide optimized back-end implementations based on:

1. Arrays for continuous indices
2. std::map for discontinuous mode

There are enough issues with the way this feature is currently implemented to warrant a complete re-work of the feature using a saner approach.

[emgre]

See also #244.

[jadamcrain]

If this is backed by std::map, you can use lower_bound when searching the map using range.

Say a master asks for points 4 - 7 but only 5 - 8 are defined. lower_bound(4) will return an iterator pointing to 5.

[jadamcrain]

Resolved in PR 296