Introduce default max number of bucket per connector

- Also add the mongodb version that stop supporting mutable
  pipelines in the changestreams, allowing to reuse a resume
  token from another one.

Issue: BB-601

extensions/oplogPopulator/constants.js

@@ -1,6 +1,11 @@
 const constants = {
     bucketMetastore: '__metastore',
     defaultConnectorName: 'source-connector',
+    // Max length in a pipeline is equal to the MongoDB BSON max document size,
+    // so 16MB. To allow for other parameters in the pipeline, we round the max
+    // to 16 MB (16777216B) / 64 (max length of a bucket name) ~= 260000
+    maxBucketPerConnector: 260000,
+    mongodbVersionWithImmutablePipelines: '6.0.0',
     defaultConnectorConfig: {
         'connector.class': 'com.mongodb.kafka.connect.MongoSourceConnector',
         'pipeline': '[]',