Merge RootCoord, DataCoord And QueryCoord into MixCoord
Make Session into one
issue : https://github.com/milvus-io/milvus/issues/37764
---------
Signed-off-by: Xianhui.Lin <xianhui.lin@zilliz.com>
issue: #29507
Notice that api_testonly.go files should be guarded by compiler tag
`test`, so that production build rules don't compile them and these APIs
don't get misused.
Signed-off-by: yiwangdr <yiwangdr@gmail.com>