Adding Initial Smapling to PingConfig

[Ahmad-2213]

When starting xray , There will be at least 2 or more requests to check the stability of outbounds, then the result can be used for burstObservatory object and leastLoad.

{
"subjectSelector":[
"outbound1",
"outbound2"
],
"pingConfig": {

   "destination": "https://connectivitycheck.gstatic.com/generate_204",
   "connectivity": "",
   "interval": "1h",
   "initialsampling": 3                 // 3 requests are sent to check the stability of outbounds.
   "sampling": 3,
  "timeout": "30s"

}
}

for instance , the result of initial sampling would be like this:
outbound1: 412ms , 354ms , 532ms
outbound2: 402ms , 384ms , 676ms

so the outbound 1 is selected as the most stable proxy.
note that after initiation, sampling would take samples and add them to previous initialsampling results.

For instance , after interval time:

outbound1: 412ms , 354ms , 532ms , 602ms
outbound2: 402ms , 384ms , 676ms , 921ms

xray will use last three samples:

outbound1: 354ms , 532ms , 602ms
outbound2: 384ms , 676ms , 921ms

Obviously , initialsampling can not be greater than sampling itself.
by doing this , the most stable outbound can be selected at the beginning of the connection.