implement a --rollback-after option, useful for speed testing
close the allocated set of descriptors passed to the 'to' endpoint child after forking to run it

this is necessary so that the SSH streams aren't held open (preventing termination in case of error).
add --only option, useful when testing
add --debug option
after a non-terminal rows command, hash the same number of rows as sent; currently this will always be one, but that's about to change
WIP, trying out a minimum block size