Steady state replication case
Suppose the following sequence occurs on the primary, which assigns recordIds as inserts come in:

• Insert {_id: 1}. Oplog entry: {op: "i", _id: 1, rid: 1}
• Delete {_id: 1}. Oplog entry: {op: "d", _id: 1, rid: 1}
• Kill and restart the primary
• Insert {_id: 2}. The primary checks on disk for the highest recordId, and that is currently 0, as no documents exist. Therefore it uses recordId(1), creating a new oplog entry: {op: "i", _id: 2, rid: 1}.

Now, if the secondary tries to apply these entries as a part of a batch, it may see in a batch:

[
  {op: "i", _id: 1, rid: 1},
  {op: "d", _id: 1, rid: 1},
  {op: "i", _id: 2, rid: 1},
]

The secondary assigns oplog entries to the applier threads based on the hash of the _id. Therefore it's possible that -
Applier thread 1 gets:

[
  {op: "i", _id: 1, rid: 1},
  {op: "d", _id: 1, rid: 1}
]

Applier thread 2 gets:

[
  {op: "i", _id: 2, rid: 1}
]

As a result it's possible for the threads to interleave in a way that we are left with data corruption (if applier thread 1 deletes document with recordId(1) after applier thread 2 has inserted).

Solutions
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