astakos: Revert bulk update of holdings in commissions

db464c6 introduced a bug: deleting holdings (and then bulk creating them)
interferes with locking; a thread that waits for holdings will not get
the deleted ones.

Revert to separate updating of holdings when issuing and resolving
commissions.
docs: Update snf-image related stuff

 * Mention the upgrade notes the fact that snf-image 0.16 is the first
   version that supports Archipelago.
 * Add instructions on how to workaround a QEMU bug found in the
   wheezy-backports package.