Fix bug: the DiscoveryRequest with resource names may never be updated if xDS server is restarted with empty SnapshotCache

[PinZhang]

The problem could be reproduced by following the steps:

• Start a xDS server and the Envoy instance
• Make some change, and push the Cluster, Listener and RouteConfiguration to the Envoy instance.
• Restart the xDS server with empty SnapshotCache, and wait for the Envoy instance connected
• Make some change again, the RouteConfiguration is never been received by the Envoy instance.

[codecov-io]

Codecov Report

Merging #103 into master will decrease coverage by 0.08%. The diff coverage is 92.3%.

@@             Coverage Diff             @@
##             master    #103      +/-   ##
===========================================
- Coverage     93.88%   93.8%   -0.09%     
- Complexity      140     141       +1     
===========================================
  Files            17      17              
  Lines           540     549       +9     
  Branches         46      47       +1     
===========================================
+ Hits            507     515       +8     
- Misses           25      26       +1     
  Partials          8       8

Impacted Files	Coverage Δ	Complexity Δ
.../io/envoyproxy/controlplane/cache/SimpleCache.java	95.61% <92.3%> (-0.58%)	30 <0> (+1)

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[PinZhang]

@joeyb Could you please review this patch? thanks.

[PinZhang]

@joeyb The patch has been updated by following your feedback, please help review again, thanks.