GC basically useless for now

```
go-ipfs version: 0.4.21-rc3-2b41ca17e-dirty
Repo version: 7
System version: amd64/darwin
Golang version: go1.12.5

ipfs daemon --enable-gc

```

ipfs gc takes too long and `StorageGCWatermark` looks like not working.
we use ipfs as a file transfer and temp file storage.
first we download files and pin it so other peers can get it immediately.
when process was done. we upload files and pin it. finally, if all process was done, we delete files we downloaded and unpin from ipfs. we cannot run `ipfs repo gc` because it will make our system frozen. So repo keeping grow until ipfs can not upload or download.
gc Enhancement is in process? or we should find other way to deal with it?