bench: pre-create domain 0 for backends

Otherwise all the domU transactions attempt to create it at the same
time with different creators, which conflicts.

Signed-off-by: David Scott <dave.scott@citrix.com>
irmin: add extra debugging around the irmin interface

Signed-off-by: David Scott <dave.scott@citrix.com>
irmin: if merge_path fails, attempt a rebase_path

This is just like using git IRL

Signed-off-by: David Scott <dave.scott@citrix.com>