The _get_ipc_attr functions should check the threshold and set ipc_type
to MPIDI_IPCI_TYPE__NONE if it doesn't meet threshold. Thus, there is no
need to return the threshold to the caller.

Not exposing threshold allows more sophiscated threshold checking if needed.

Slight refactor to streamline the do_ipc check.

We no longer use the status and the ipc fields. Remove.

Move relevant code together.

Check MPIR_CVAR_CH4_IPC_GPU_HANDLE_CACHE at the caller place rather than
inside ipc_handle_cache_search and MPIDI_GPU_ipc_handle_cache_insert.
This keeps the search and insert utility functions simple, and avoids
the extra insert_successful check.

Also, move MPIDI_GPU_ipc_handle_cache_insert into the branch to avoid
extra checking and setting handle_status.

Store MPL_gpu_ipc_mem_handle_t directly rather than
MPIDI_IPCI_ipc_handle_t. Add separate utilities to avoid mixing
different requirement of caching local ipc handles versus caching remote
mapped addresses.

Directly cache mapped_base_addr.

Refactor and move the code into MPIDI_GPU_copy_data. The gpu related
code belongs in gpu_post.c.

Make ipc attr query and ipc handle preparation orthogonal and use
separate data structure to avoid complication. In attr query, we focus
on whether we should enable IPC, and only cache necessary information
locally that is sufficient for preparing the handle. All the actual
handle preparation logic moved to the separate function later.

Different IPC features have different logic. Move most of the special
data structure and logic inside the feature code to avoid changes in one
feature affecting the code in other features.

ch4/ipc: refactor fixup

This is cleaner.

The xpmem functions are now always guarded by #ifdef
MPIDI_CH4_SHM_ENABLE_XPMEM.