[SPARK-3889] Attempt to avoid SIGBUS by not mmapping files in ConnectionManager

In general, individual shuffle blocks are frequently small, so mmapping them often creates a lot of waste. It may not be bad to mmap the larger ones, but it is pretty inconvenient to get configuration into ManagedBuffer, and besides it is unlikely to help all that much.
i think the old code had a config parameter for using mem map or this ..