irmin: use an in-memory database if we don't provide --database

Signed-off-by: David Scott <dave.scott@citrix.com>
xenstored: add a '--prefer-merge' flag

The default is to use rebase, which is fast but doesn't produce
pretty histories. If you prefer the pretty histories then please
note that it's currently not as efficient when run in parallel.

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