Legion: mixing user-defined and Legion-generated layout constraint IDs

[mariodirenzo]

Right now, if an application is registering layout constraints both with user-defined and Legion-generated IDs, two constraints may be defined with the same ID. To avoid this situation, it would be useful to distinguish the pool of values that a user can pick for the constraint IDs from the values that Legion can automatically generate.

@elliottslaughter, please add this issue to #1032

[lightsighter]

@mariodirenzo Pull and try again with either the master or control replication branches.

[mariodirenzo]

It works! Thank you for the quick merge of the feature.