When MC_STORE_MEMCPY is not explicitly set, auto-detect based on
installed transports: enable memcpy only when TCP is the sole
transport (no RDMA, NVLink, etc.), since TCP loopback is less
efficient than direct memcpy for same-host transfers. In RDMA
environments the default remains disabled, as RDMA is more
resource-efficient.
Add TransferEngine::isTcpOnly() API that checks whether TCP is the
only installed transport via MultiTransport::transport_map_. This
is future-proof: any new transport registered via installTransport()
is automatically accounted for without maintaining a protocol list.
TENT path returns false unconditionally since TENT already rejects
TCP loopback without MC_STORE_MEMCPY.
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
38c3975138