Advanced publish/subscribe functionality

[x] rmw_publish - Same as above. Uses allocator specified in allocation instead of publisher's allocator
[ ] rmw_publish_serialized_message - Same as above but serialized
[x] rmw_take - Reads available message and copies it into provided pointer. Wrapper for taken_loaned and return_loaned.
[x] rmw_take_with_info - Like above, but more so
[x] rmw_take_sequence - Like above, but multiple messages
[ ] rmw_take_serialized_message - Like above, but serialized
[ ] rmw_take_serialized_message_with_info - Like above, but more so
[ ] rmw_subscription_set_content_filter - You can do this?
[ ] rmw_subscription_get_content_filter - Same question
[ ] rmw_init_publisher_allocation - This either creates our allocators, or goes unused
[ ] rmw_fini_publisher_allocation - Destroy what's created above (unmap, etc)
[ ] rmw_init_subscription_allocation - Allocator preference is specified
[ ] rmw_fini_subscription_allocation - Cleanup from above

Unit tests from test_rmw_implementation to pass

nightduck / rmw_hazcat