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`collection_name` is the name of colletion the collection to delete entities from.

`expr` is an expression indicated whether an entity should be deleted in the collection. Only the `in` operator is supported in the Delete API. Document of expression: https://github.com/milvus-io/milvus/blob/master/docs/design_docs/query_boolean_expr.md

`partition_name` is the name of the partition to delete entities from, `None` means all partition.

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Same as Insert API, Milvus only guarantee the visibility of operations with a one client. This means that, within the sequence of the operations "delete(), search()", the result of the search will not contains the entities deleted. Since different clients connect to different Proxy, the time between different Proxy is not exactly th the same. So even if you manually call the delete method and the search method on two clients sequentially, it is uncertain whether the search request returns the deleted entities.

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1. Insert channel, handle following msg types:
   1. DDL msg (CreateCollection/DropCollection/CreatePartition/DropPartition)
   2. InsertMsg
   3. DeleteMsg
2. Search Query channel, handle following msg types:
   1. SearchMsg
   2. RetrieveMsg

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QueryNode consumes messages from both Insert channel and Search Query channel.

To support delete, we will send DeleteMsg into Insert channel also.

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Since Milvus's storage is an append-only, `delete` function is implemented using soft delete, setting a flag on entity to indicate this entity has been deleted.

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Proposal delete operation persistent

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1. Proxy receives a delete request
   1. parse `expr` to get primary keys, then split DeleteMsg into insert channels by primary keys (done)
2. DataNode receives a delete request from the insert channel, save it in buffer, and write it into the delta channel
   1. save deleted id into structure map<segID, list(id, ts)>
   DataNode receives a flush request, write out the deletions saved abovedeleted ids are saved into a watches insert channelfinish
   1. receive DeleteMsg and persist delete data
      1. when DataNode start up, load all segments info from Minio into memory (done)
      2. update datanode flowgraph
         1. when DDNode receive DeleteMsg, save it into FlowGraphMsg structure, and send FlowGraphMsg to next node InsertBufferNode (done)
         2. when InsertBufferNode receive FlowGraphMsg, process InsertMsg in it, then wrapper DeleteMsg into another FlowGraphMsg and sent it to next node DeleteNode (done)
         3. when DeleteNode receive FlowGraphMsg, process DeleteMsg in it, save deleted ids and timestamps into delBuf (done)
      3. update DeleteNode to handle flushMsg
         1. add another flushChan for DeleteNode (done)
         2. when DeleteNode receives flushMsg, save all data in delBuf into MinIO 
            1. deleted ids and timestamps are saved into two separated Minio file with name "/by-dev/deltalog/collectionID/partitionID/segmentID/xxx"
3. DataNode notifies IndexNode to building indexes
4. 1. 1. 1. 1. (done)
   2.  DataNode forwards ttMsg and DeleteMsg to delta channel (doing)

Proposal delete operation serving search(sealed+growing)

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