Solr redundant-write solution

This patch allows ezfind to update a list of solr instances at once, so this is basically a distributed-write-to-solr modification.

This is a key part of a solution that removes Solr as a single point of failure. Specifically, this allows eZ Publish to write to a each of a pool of instances, and read from them via a VIP or load balancer. This avoids the issues the Solr has with peer to peer replication, but more importantly provides support for enterprise clients that are using older versions of Solr.