add --resume option to osm import-history
reuse FindTreeChild command while looking for way nodes
prepare FindTreeChild with current head tree to avoid multiple lookups
Do not load all buckets to compute the final tree size/num trees.

RevTreeBuilder.normalizeToBuckets() performance optimization to avoid loading all the
buckets that compose the tree unnecessarily to compute the final tree size and number
of subtrees, but instead pre compute the deltas out of the changed buckets only.
do now swallow exception in ImportOp