fix the deadlock when closing db

[KANIOYH]

Problem

find in bench_test.go: BenchmarkGet
db.Close() hold db.mu and close flushChan.

db.mu.Lock()
defer db.mu.Unlock()
close(db.flushChan)
<-db.closeChan

At the same time, the listenMemtableFlush -> db.flushMemtable may trying hold db.mu to flush memtable.

...
// delete old memtable kept in memory
db.mu.Lock()
if table == db.activeMem {
...

listenMemtableFlush blocked, it cannot send msg to closeChan, and db.Close() blocked in <-db.closeChan.

Solve

close flushChan without db.mu.

close(db.flushChan)
<-db.closeChan
db.mu.Lock()
defer db.mu.Unlock()