ch4/ipc: default MPIR_CVAR_CH4_IPC_XPMEM_P2P_THRESHOLD to 64K

Small buffers are less likely be used persistently. XPMEM's mapping cost
will slow down the intranode message performance unless the buffer is
persistent and the mapping cost get amortized.

Apply a larger threshold to avoid slow down applications that use ad-hoc
memory for small messages.

Author: hzhou

src/mpid/ch4/shm/ipc/xpmem/xpmem_post.h

@@ -26,7 +26,7 @@
     - name        : MPIR_CVAR_CH4_IPC_XPMEM_P2P_THRESHOLD
       category    : CH4
       type        : int
-      default     : 1024
+      default     : 65536
       class       : none
       verbosity   : MPI_T_VERBOSITY_USER_BASIC
       scope       : MPI_T_SCOPE_ALL_EQ