Improve efficiency of `DatastoreIndexingRouter#source_event_versions_in_index`: leverage routing/timestamp

[myronmarston]

This method searches all shards an indices:

elasticgraph/elasticgraph-indexer/lib/elastic_graph/indexer/datastore_indexing_router.rb

Lines 173 to 180 in 89b5446

	# Note: we intentionally search the entire index expression, not just an individual index based on a rollover timestamp.
	# And we intentionally do NOT provide a routing value--we want to find the version, no matter what shard the document
	# lives on.
	#
	# Since this `source_event_versions_in_index` is for handling malformed events, its possible that the
	# rollover timestamp or routing value on the operation is wrong and that the correct document lives in
	# a different shard and index than what the operation is targeted at. We want to search across all of them
	# so that we will find it, regardless of where it lives.

This is quite inefficient, particularly when you have many malformed events--it can cause significant load problems on a cluster.

The rationale given in that comment is speculative and I suspect we've never actually had malformed documents with the wrong routing/index rollover timestamp. We should make it more efficient.