use sync.Pool to optimize db.Put operation

[roseduan]

Now the db.Put operation will create a new batch every time, which may cause too much memory allocation.
We can use sync.Pool to avoid this.

The goleveldb has done this, see https://github.com/syndtr/goleveldb/blob/master/leveldb/db_write.go#L362

[Yikun Chen]

I will finish it.

[Jeremy]

Now the db.Put operation will create a new batch every time, which may cause too much memory allocation. We can use sync.Pool to avoid this.

The goleveldb has done this, see https://github.com/syndtr/goleveldb/blob/master/leveldb/db_write.go#L362

That's a good idea

[roseduan]

Thanks, I have done this in commit 02c9953

you can add it to lotusdb if you want.