Reduce the DiskIoMgr thread parameters for filesystems that are not being tested

DiskIoMgr is using a large number of threads, most of which are idle. This makes minidumps/cores larger and slower to process. For developers, this can slow down gdb attach and navigating through the threads.

For our test infrastructure / development, we can reduce the threads for filesystems that are not the TARGET_FILESYSTEM. If we aren't testing ABFS, then we don't need the full set of ABFS threads. In general, we can reduce almost all of the non-TARGET_FILESYSTEM DiskIoMgr thread counts to 1.

We should have a way to turn it off, but it seems like this could be the default mode.