Support flushall/flushdb option

[shangxiaoxiong]

When flushall or flushdb is executed, the disk is still occupied until a compaction occurs. this may bring confusion to users because to free disk space quickly is expected in most case. this commit makes kvrocks support flushall/flushdb with option which decides whether to compact immediately. Note that compaction caused by flushall/flushdb will spread to slave.

supported options:

• sync: delete data and compact immediately
• async: delete data
• none: the same with sync

[shangxiaoxiong]

l found a bug through test and l am locating

[ShooterIT]

@shangxiaoxiong solved?

[tisonkun]

Closed as stale.

Codebase evolves a lot since the last activities on this patch. @shangxiaoxiong you're welcome to propose the changes again based on the nightly unstable branch.