add locking to pod cache (#1379)
add locking to pod cache (#1379)

Automatic merge from submit-queue

add locking to pod cache

**What this PR does / why we need it**:
PodCache has no mutex protecting its internal state. Add RWMutex to fix this.

**Which issue this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close that issue when PR gets merged)*: fixes #1377 

**Special notes for your reviewer**:

**Release note**:

```release-note
Fix race in Pilot PodCache
```
vadimeisenbergibm/pilot,istio/old_pilot_repo,vadimeisenbergibm/pilot,istio/old_pilot_repo,vadimeisenbergibm/pilot,istio/old_pilot_repo