server: add a snapshot collecting DiscoveryServerCallbacks

[snowp]

Adds a callback that watches the streams for requests and stream closes and cleans up unreferenced snapshots after some time period.

This upstreams the GC mechanism we've been using internally for a few months.

Signed-off-by: Snow Pettersen snowp@squareup.com

[codecov-io]

Codecov Report

Merging #73 into master will increase coverage by 0.09%. The diff coverage is 95.55%.

@@             Coverage Diff              @@
##             master      #73      +/-   ##
============================================
+ Coverage     94.52%   94.62%   +0.09%     
- Complexity      115      130      +15     
============================================
  Files            12       13       +1     
  Lines           457      502      +45     
  Branches         39       44       +5     
============================================
+ Hits            432      475      +43     
  Misses           18       18              
- Partials          7        9       +2

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/envoyproxy/controlplane/cache/Snapshot.java	100% <100%> (ø)	23 <1> (+1)	:arrow_up:
...ne/server/callback/SnapshotCollectingCallback.java	95.34% <95.34%> (ø)	14 <14> (?)

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[snowp]

Open to integrating this more with the DiscoveryServer, for now this just adds the callback itself. I can imagine something like DiscoveryServer.createWithCollector(cache, collectAfterMinutes)

[snowp]

ping @joeyb