rocksdb: configurable MaxOpenFiles

[JayT106]

#218 made a change for setup MaxOpenFiles to 4096, which makes the RAM usage could easily go over 64GB when our application doing a snapshot or the genesis state export that requires the DB iterating. We tried to reduce it to 1024 and it turns out we can run it within 30GB.

The setup here

tm-db/rocksdb.go

Line 45 in d1b9b74

opts.OptimizeLevelStyleCompaction(512 * 1024 * 1024)

made each sst file target size equal to 64MB(if we look at how rocksdb decided the file size), therefore I am not sure to budget 2MB for each opened file is base on which file size?

We should consider the node's capability and decide which number is proper for running the rocksdb instead of a fixed number.

ref:
https://github.com/facebook/rocksdb/wiki/Memory-usage-in-RocksDB