[todo] rocksdb tuneup. for chaindb storage scalability

[trustfarm-dev]

openethereum/parity-ethereum#9033

actual
Blockheights 58986xxx , FULL Archive mode sync : Storage is 1.24TB in 2TB SSD.
It is increasing very stiff. when 2TB SSD will be full soon.

suggestion
For solve this kind of storage scalability , I suggest parity use multiple [dbpaths storage options provided by rocksdb.](https://github.com/facebook/rocksdb/blob/17339dc2f39a87a9cf097e056a59560b0795ff6e/include/rocksdb/options.h#L468 ,"db_paths options")

  // For example, you have a flash device with 10GB allocated for the DB,
  // as well as a hard drive of 2TB, you should config it to be:
  //   [{"/flash_path", 10GB}, {"/hard_drive", 2TB}]

Because latest fast block importing and synchronizing is done in SSD , old and infrequent referencing block data will be stored in HDD.

It will helpful, shorterm storage scalability.