btl/base: add a subsystem for supporting active-message "RDMA"

This commit adds a new subsystem to the btl base: active-message
RDMA. The subsystem can be activated on any non-RDMA or partial-
RDMA btl (for example tcp) by calling:

```c
mca_btl_base_am_rdma_init(mca_btl_base_module_t *btl).
```

Once called the btl module supplied with be augmented to have
btl_get, btl_put, btl_fop, and btl_cswap functions. Flags will
be set to indicate that the BTL is using active-message
versions of these functions.

Calling this function on a btl will register a progress
function for handling retries of intiator or target-side
operations. There may be some overhead once the progress
function is registered.

Active-message "RDMA" support is not activated on any BTL
module by default.

Signed-off-by: Nathan Hjelm <hjelmn@google.com>

opal/mca/btl/base/Makefile.am

@@ -9,6 +9,8 @@
 #                         University of Stuttgart.  All rights reserved.
 # Copyright (c) 2004-2005 The Regents of the University of California.
 #                         All rights reserved.
+# Copyright (c) 2021      Triad National Security, LLC. All rights reserved.
+# Copyright (c) 2021      Google, LLC. All rights reserved.
 # $COPYRIGHT$
 #
 # Additional copyrights may follow
@@ -20,10 +22,12 @@ dist_opaldata_DATA = base/help-mpi-btl-base.txt
 
 headers += \
         base/base.h \
-        base/btl_base_error.h
+        base/btl_base_error.h \
+        base/btl_base_am_rdma.h
 
 libmca_btl_la_SOURCES += \
         base/btl_base_frame.c \
         base/btl_base_error.c \
         base/btl_base_select.c \
-        base/btl_base_mca.c
+        base/btl_base_mca.c \
+        base/btl_base_am_rdma.c

opal/mca/btl/base/base.h

@@ -11,6 +11,7 @@
  *                         All rights reserved.
  * Copyright (c) 2006      Sun Microsystems, Inc.  All rights reserved.
  * Copyright (c) 2008      Cisco Systems, Inc.  All rights reserved.
+ * Copyright (c) 2020      Google, LLC. All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -28,6 +29,10 @@
 #include "opal/mca/mca.h"
 #include "opal/mca/btl/btl.h"
 
+#define MCA_BTL_BASE_TAG_RDMA      MCA_BTL_TAG_BTL_BASE
+#define MCA_BTL_BASE_TAG_ATOMIC    (MCA_BTL_TAG_BTL_BASE + 1)
+#define MCA_BTL_BASE_TAG_RDMA_RESP (MCA_BTL_TAG_BTL_BASE + 2)
+
 BEGIN_C_DECLS
 
 struct mca_btl_base_selected_module_t {