Add verify support

[thebrianchen]

Port from JS.

[codecov]

Codecov Report

Merging #1107 into master will increase coverage by 0.01%.
The diff coverage is 33.33%.

@@             Coverage Diff              @@
##             master    #1107      +/-   ##
============================================
+ Coverage     53.37%   53.38%   +0.01%     
- Complexity     6400     6410      +10     
============================================
  Files           719      720       +1     
  Lines         34853    34870      +17     
  Branches       4624     4626       +2     
============================================
+ Hits          18603    18616      +13     
- Misses        14769    14774       +5     
+ Partials       1481     1480       -1

Impacted Files	Coverage Δ	Complexity Δ
...om/google/firebase/firestore/core/Transaction.java	0% <0%> (ø)	0 <0> (ø)	⬇️
...ebase/firestore/model/mutation/VerifyMutation.java	30.76% <30.76%> (ø)	2 <2> (?)
...le/firebase/firestore/remote/RemoteSerializer.java	79.31% <66.66%> (+0.69%)	147 <0> (+4)	⬆️
...google/firebase/database/snapshot/BooleanNode.java	84.61% <0%> (-7.7%)	12% <0%> (-1%)
...a/com/google/firebase/database/core/WriteTree.java	76.24% <0%> (-1.11%)	53% <0%> (-1%)
...va/com/google/firebase/database/core/SyncTree.java	62.25% <0%> (+0.25%)	40% <0%> (ø)	⬇️
...n/java/com/google/firebase/storage/UploadTask.java	72.52% <0%> (+1.09%)	47% <0%> (+1%)	⬆️
...irebase/firestore/model/mutation/Precondition.java	57.14% <0%> (+5.71%)	19% <0%> (+5%)	⬆️

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[wilhuff]

LGTM