Support pause/resume of compaction

When there are a lot of documents (more than 100M) in a ForestDB instance, a compaction process will take very long time; from tens of minutes to several hours. If ForestDB is shut down during the compaction, than all new documents will be moved from the new (compacted) file to the old file on the next open of the ForestDB handle, and we have to start over the compaction again. This can be a lot of stress.

If ForestDB supports pause/resume of compaction, then we can be free from the unnecessary overhead.

Furthermore, we can do a sort of load balancing; when there are heavy KV operations from users, compaction process may be paused temporarily for performance issue. And we can resume the compaction again when the load becomes idle.