Mutable repair: use new MODE_REPAIR to query all servers *and* get privkey

Mutable repair: use new MODE_REPAIR to query all servers *and* get privkey

This fixes bug #1689. Repair was using MODE_READ to build the servermap,
which doesn't try hard enough to grab the privkey, and also doesn't guarantee
sending queries to all servers. This patch adds a new MODE_REPAIR which does
both, and does a separate, distinct mapupdate to start wth repair cycle,
instead of relying upon the (MODE_CHECK) mapupdate leftover from the
filecheck that triggered the repair.