Reduce the overhead of UCX add_procs with intercommunicators

 * When creating a large number of intercommunicators with `MPI_Intercomm_create`
   the UCX pml add_procs routine is called for each "new" process. This results
   in a call to `ucp_ep_create` and overwrites the old endpoint at the PML level
   if there was already one in place. However, it adds a new endpoint to the UCX
   instance below without removing the old endpoint. This results in accumulating
   a large number of endpoints paired with the UCX worker. Creating the endpoints
   has overhead which contributes to the slowdown for the `MPI_Intercomm_create`
   function.
 * On Finalize cleaning these up endpoints occurs in the `ucp_worker_destroy`
   function. Since there are a signifiant number of endpoints it takes quite a
   while to cleanup.
 * In this patch, we first check to see if an endpoint has already been created
   for this process. If so then we skip adding it again. Otherwise we create a
   new endpoint.

Signed-off-by: Joshua Hursey <jhursey@us.ibm.com>

Author: jjhursey

ompi/mca/pml/ucx/pml_ucx.c

@@ -410,6 +410,11 @@ static ucp_ep_h mca_pml_ucx_add_proc_common(ompi_proc_t *proc)
     ucp_ep_h ep;
     int ret;
 
+    /* Do not add a new endpoint if we already created one */
+    if (NULL != proc->proc_endpoints[OMPI_PROC_ENDPOINT_TAG_PML]) {
+        return proc->proc_endpoints[OMPI_PROC_ENDPOINT_TAG_PML];
+    }
+
     ret = mca_pml_ucx_recv_worker_address(proc, &address, &addrlen);
     if (ret < 0) {
         return NULL;