Update Export workers to read from replica

What does this MR do?

This MR updates Project/Group Export workers to use replica db when performing export.

Few notes on the implementation approach:

1. data_consistency :delayed option is available https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/workers/concerns/worker_attributes.rb#L84 but it only works with workers that are retriable, which project export workers are not. They were made not retriable in !35344 (merged) since data showed that export worker retries are wasteful and generally do not make a difference in the end result. I tried reintroducing retries back to the export worker and using delayed approach but reads were still going to primary #338326 (comment 650801991)
2. data_consistency :sticky is not an option since it keeps connection to a replica but then switches to primary on first read, but does not switch back to replica on the reads after

This change should be applicable to:

• ProjectExportWorker
• GroupExportWorker
• RelationExportWoker

since they all use streaming serializer.

Example of export hitting replica with proposed change:

{
  "severity": "INFO",
  "time": "2021-08-13T14:52:21.273Z",
  "class": "ProjectExportWorker",
  "args": [
    "1",
    "52"
  ],
  ...
  "meta.feature_category": "importers",
  "correlation_id": "f4ae54ac9744a72e390a38a1a2a22abf",
  "idempotency_key": "resque:gitlab:duplicate:project_export:46746c03a500f8babbf198679902292e0e870bd8fc76cd1ce92246ddf0c66d56",
  "worker_data_consistency": "always",
  "enqueued_at": "2021-08-13T14:50:19.263Z",
  "job_size_bytes": 6,
  "pid": 40616,
  "message": "ProjectExportWorker JID-8aa75e94765f61aad3f4e0e1: done: 122.008985 sec",
  "job_status": "done",
...
  "db_count": 4992,
  "db_write_count": 17,
  "db_cached_count": 401,
  "db_replica_count": 4956,
  "db_replica_cached_count": 401,
  "db_replica_wal_count": 0,
  "db_replica_wal_cached_count": 0,
  "db_primary_count": 36,
  "db_primary_cached_count": 0,
  "db_primary_wal_count": 0,
  "db_primary_wal_cached_count": 0,
  "db_replica_duration_s": 7.822,
  "db_primary_duration_s": 0.061,
  "cpu_s": 81.000855,
  "duration_s": 122.008985,
  "completed_at": "2021-08-13T14:52:21.273Z",
  "load_balancing_strategy": "primary",
  "db_duration_s": 4.462793
}

Mentions #338326 (closed)

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How to setup and validate locally (strongly suggested)

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