tdb: add -k option to tdbtorture

tdb: add -k option to tdbtorture

To test the case of death of a process during transaction commit, add
a -k (kill random) option to tdbtorture.  The easiest way to do this
is to make every worker a child (unless there's only one child), which
is why this patch is bigger than you might expect.

Using -k without -t (always transactions) you expect corruption, though
it doesn't happen every time.  With -t, we currently get corruption but
the next patch fixes that.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>