Send EDS response after CDS even if EDS hasn't change

[lukidzi]

On our development environment we found that if snapshot already exists and new Envoy connects to EC it might respond immediately in createWatch method. Method setSnapshot can ensure that EDS will be sent after CDS but here we can't be sure that. I've tested this with ADS mode and also made changes for this mode.

This is how it looks like:

1. Envoy connects to control-plane
2. Snapshot already exists in control-plane <- other instance share same group
3. Control-plane respond with CDS in createWatch method
4. There is snapshot update which change CDS and EDS versions
5. Envoy sends EDS request
6. Control-plane respond with EDS in createWatch method
7. Envoy resume CDS and EDS requests.
8. Envoy sends request CDS
9. Control plane respond with CDS in createWatch method
10. Envoy sends EDS requests
11. Control plane doesn't respond because version hasn't changed
12. Cluster of service stays in warming phase

I've prepared a change which sets flag hasClusterChanged after CDS response. Because it will be shared between threads I made it volatile. I would appreciate hearing your thoughts on this.

[codecov-io]

Codecov Report

Merging #131 into master will increase coverage by 0.20%. The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #131      +/-   ##
============================================
+ Coverage     89.56%   89.77%   +0.20%     
- Complexity      195      202       +7     
============================================
  Files            22       22              
  Lines           604      616      +12     
  Branches         49       51       +2     
============================================
+ Hits            541      553      +12     
  Misses           48       48              
  Partials         15       15              

Impacted Files	Coverage Δ	Complexity Δ
.../io/envoyproxy/controlplane/cache/SimpleCache.java	84.00% <100.00%> (+0.80%)	36.00 <2.00> (+4.00)
...o/envoyproxy/controlplane/cache/TestResources.java	97.77% <100.00%> (+0.02%)	9.00 <1.00> (+1.00)
...lane/server/AdsDiscoveryRequestStreamObserver.java	100.00% <100.00%> (ø)	14.00 <0.00> (+2.00)
...olplane/server/DiscoveryRequestStreamObserver.java	85.54% <100.00%> (+0.17%)	20.00 <1.00> (ø)

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