Promote domains and Cyclone library to entities

This commit adds two entity types: a "domain", which is the parent of
participants and which is instantiated for each domain that has at least
one participant in it; and "cyclonedds", which is a representation of
the (initialized) Cyclone DDS library in the process and that is the
parent of all domain entities.  The handle of the latter is a
compile-constant, DDS_CYCLONEDDS_HANDLE.

This changes the return value from dds_get_parent when executed on a
participant: it now returns the handle of the entity representing the
domain the participant is attached to.  Two participants in the same
domain self-evidently return the same domain entity.

This allows deleting all participants in a domain by calling dds_delete
on the domain entity, or tearing down everything and deinitializing the
library by calling dds_delete on the top-level entity.

Signed-off-by: Erik Boasson <eb@ilities.com>

src/core/ddsc/include/dds/dds.h

@@ -66,6 +66,9 @@ struct ddsi_serdata;
 #define DDS_BUILTIN_TOPIC_DCPSSUBSCRIPTION ((dds_entity_t) (DDS_MIN_PSEUDO_HANDLE + 4))
 /** @}*/
 
+/** Special handle representing the entity corresponding to the CycloneDDS library itself */
+#define DDS_CYCLONEDDS_HANDLE              ((dds_entity_t) (DDS_MIN_PSEUDO_HANDLE + 256))
+
 /** @name Communication Status definitions
   @{**/
 typedef enum dds_status_id {
@@ -860,7 +863,8 @@ dds_get_children(dds_entity_t entity, dds_entity_t *children, size_t size);
  * DataReaders), etc are also attached to that domain.
  *
  * This function will return the original domain ID when called on
- * any of the entities within that hierarchy.
+ * any of the entities within that hierarchy.  For entities not associated
+ * with a domain, the id is set to DDS_DOMAIN_DEFAULT.
  *
  * @param[in]  entity   Entity from which to get its children.
  * @param[out] id       Pointer to put the domain ID in.

src/core/ddsc/include/dds/ddsc/dds_public_impl.h

@@ -108,7 +108,9 @@ typedef enum dds_entity_kind
   DDS_KIND_COND_READ,
   DDS_KIND_COND_QUERY,
   DDS_KIND_COND_GUARD,
-  DDS_KIND_WAITSET
+  DDS_KIND_WAITSET,
+  DDS_KIND_DOMAIN,
+  DDS_KIND_CYCLONEDDS
 } dds_entity_kind_t;
 
 /* Handles are opaque pointers to implementation types */