Race condition for coordinator node init

From a unit test case that issue concurrent select queries to coordinator nodes, it’s found that there could be 3 race condition issues:

1. If multiple concurrent requests find the synthetic collection is not yet created, they might all attempt to create the synthetic collection. This could trigger SolrException on `collection already exists`

2. Similarly, if multiple concurrent requests find there’s no replica of the synthetic collection for current node (multiple coordinator node scenario), then CoordinatorHttpSolrCall#addReplica could be invoked multiple times. This should not trigger any exception, but would create multiple replicas for the same node in the synthetic collection

3. The existing logic here assumes if syntheticColl.getReplicas(solrCall.cores.getZkController().getNodeName()) returns non empty result, then the following call in here should return a core. Unfortunately, the first call can return a non empty list but with a DOWN replica if another request is in the progress of creating such replica. In this case, the solrCall.getCoreByCollection(syntheticCollectionName, isPreferLeader) would call super.getCoreByCollection at here which would return a null (since super impl only returns active replica). So CoordinatorHttpSolrCall#getCoreByCollection would end up calling CoordinatorHttpSolrCall#getCore , introducing an infinite loop and cause stack overflow