compaction bandwidth limit

Compaction runs to its full speed. This change provides two knobs to limit the compaction bandwidth:
1 compactBwLimit: the desired max bandwidth per compaction. If the compaction thread runs too fast, it will be put to sleep for a while;
2. numOfFilesDisableCompactLimit: Limiting the compaction speed will increase the # of files per store which increases the read latency. This parameter can disable the compaction limit when the number of files in a store exceeds the value.