Modify DADI's dadi-p2proxy to support for FaaSNet

My understanding is as follows.

DADI's p2proxy supports downloading back to the source(image registry). For example, vm2 sends a layer block request to vm1, if vm1 has no the block, finally it can get the block from registry.

As for FaaSNet' p2p, assuming there is such a pipeline: registry->vm0->vm1->vm2. So vm2 sends a layer block request to vm1,
vm1 will only responds to the request after downloading from VM0.

So to support for FaaSNet, i use golang's conditional variable to build sync lib.

```
import (
"sync"
)

type Synclib interface {
Wait(key string)
Broadcast(key string)
}

type syCache struct {
lock sync.Mutex
waitMap map[string]*sync.Cond
broadcastMap map[string]struct{}
}

// NewSyCache
func NewSyCache() Synclib {
return &syCache{
waitMap: make(map[string]*sync.Cond),
broadcastMap: make(map[string]struct{}),
}
}

// key := filepath.Join(r.path, strconv.FormatInt(offset, 10))
func (sc *syCache) Wait(key string) {
sc.lock.Lock()
defer sc.lock.Unlock()

if _, ok := sc.broadcastMap[key]; ok {
return
}

val, ok := sc.waitMap[key]

if ok {
val.Wait()
return
}

// first one
cond := sync.NewCond(&sc.lock)
sc.waitMap[key] = cond

cond.Wait()
}

func (sc *syCache) Broadcast(key string) {
sc.lock.Lock()
defer sc.lock.Unlock()

sc.broadcastMap[key] = struct{}{}

val, ok := sc.waitMap[key]

if !ok {
return
}

val.Broadcast()

delete(sc.waitMap, key)
}

```
When vm1's p2proxy server receives vm2's request, if vm1 has no the layer block, vm1's p2pHandler goroutineA will invoke Wait() until vm1 `fileCacheItem.Fill(fetch)` from vm0. After fetching the layer block, vm1's goroutineB will invoke `Broadcast()` to wake up goroutineA.

My problem is Creating container takes too long.
Assuming there is such a pipeline.
```
----> vm2
registry----->vm1
----> vm3

```
vm1 took 5191ms(commonly takes 2200ms from registry) to create a container, and vm2 and vm3 took 14520ms to create the same container. `time sudo nerdctl run --snapshotter=overlaybd xxx`

I cat `/var/log/overlaybd.log` and find that vm2 and vm3 open dev took ~1700ms(log output).
I have no idea what causes the problem.