Feature Request: support different prefix_extractor for every iteration

[lhsoft]

Currently rocksdb just support prefix_extractor for cf.
But in our scenario we need different prefix_extractor for different iteration.
For example, we have following data
1_1_1
1_1_2
1_2_1
1_2_2
2_1_1
2_1_2
...
We need to scan data with prefix 1_1_ and at the same time, we also need to scan data with prefix 1_.
I would like to work on it and send the patch out later.

[zaidoon1]

i believe what you are looking for is #12824