Added file transfer throttling, configured with FILE_TRANSFER_DISK_LOAD_THROTTLE. #3613

WARNING WARNING WARNING: I changed the file transfer protocol.  It does a version check
to see if the peer is >= 8.1.0.  If it is, it uses the new protocol.  This means older
builds of 8.1.0 are incompatible with 8.1.0 from here on.