Converting the same msgposition's vchannel to a pchannel multiple times
would result in an invalid pchannel, leading to seek failure and panic.
This PR:
1. Make a copy of msgposition in msgdispatcher.
2. Check if channel is already a pchannel, no further channel conversion
is performed.
issue: https://github.com/milvus-io/milvus/issues/34221
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Signed-off-by: bigsheeper <yihao.dai@zilliz.com>