Use L1 size as estimate for L0 size in LevelCompactionBuilder::GetPathID

Use L1 size as estimate for L0 size in LevelCompactionBuilder::GetPathID

Summary:
Fix for [2461](https://github.com/facebook/rocksdb/issues/2461).

Problem: When using multiple db_paths setting with RocksDB, RocksDB incorrectly calculates the size of L1 in LevelCompactionBuilder::GetPathId.

max_bytes_for_level_base is used as L0 size and L1 size is calculated as (L0 size * max_bytes_for_level_multiplier). However, L1 size should be max_bytes_for_level_base.

Solution: Use max_bytes_for_level_base as L1 size. Also, use L1 size as the estimated size of L0.
Closes https://github.com/facebook/rocksdb/pull/2903

Differential Revision: D5885442

Pulled By: maysamyabandeh

fbshipit-source-id: 036da1c9298d173b9b80479cc6661ee4b7a951f6