copy store through a temporary file

Still waiting for CI, but feel free to take a look already.

Tested by manually removing arakoon_0.db from a node and then starting it.
Resulted in the following output:
Jul 19 15:34:58 0928: (main|debug): stat /tmp/arakoon/arakoon_0/head.db
Jul 19 15:34:58 0930: (main|debug): stat /tmp/arakoon/arakoon_0/arakoon_0.db
Jul 19 15:34:58 0931: (main|debug): stat /tmp/arakoon/arakoon_0/arakoon_0.db.tmp
Jul 19 15:34:58 0931: (main|debug): copy_file /tmp/arakoon/arakoon_0/head.db /tmp/arakoon/arakoon_0/arakoon_0.db.tmp
Jul 19 15:34:58 1110: (main|debug): done: copy_file
Jul 19 15:34:58 1111: (main|debug): rename /tmp/arakoon/arakoon_0/arakoon_0.db.tmp -> /tmp/arakoon/arakoon_0/arakoon_0.db