Performance optimization

[wsy19961129]

1. For uploading data, use the insert operation only
   1. For each file, create a separate data clip;
   2. Uploading data will not replace the unversioned data clips instead of inserting directly; this is to avoid extra cost by using find_and_update during uploading;
2. For querying data
   1). For querying files, need to add an upper limit of queried file per API; the frontend needs to dynamically load data instead of loading all at once. The search input now needs to obtain data from real-time result instead from the previous API cache; the source of the statistics need to be redesigned;
   2). For querying variables, need to store the latest data in a static way.