Repair should never reuse a tree

AEService currently 'caches' MerkleTrees that have been generated by the local node, and can respond to a request for a tree with a cached version. This means that despite possibly having performed a repair, the two nodes will still be holding cached trees, and can respond to one another with the cached version, resulting in redundant repairs.

Rather than a cache, the map of trees is intended to be a rendezvous point for two nodes to exchange relatively up-to-date trees. A map still might work, but other synchronization objects should be explored.