fixes #726

[GavinChenYan]

add fix for two places:

1. for service lookup, always use lastIndex as 0L to avoid consul blocking queries.
2. Implementations must check to see if a returned index is lower than the previous value, and if it is, should reset index to 0

[codecov-commenter]

Codecov Report

Merging #727 into 1.6.x will increase coverage by 0.00%.
The diff coverage is 25.00%.

@@            Coverage Diff            @@
##              1.6.x     #727   +/-   ##
=========================================
  Coverage     49.87%   49.87%           
- Complexity     1921     1923    +2     
=========================================
  Files           261      261           
  Lines         11750    11756    +6     
  Branches       1667     1670    +3     
=========================================
+ Hits           5860     5863    +3     
- Misses         5194     5198    +4     
+ Partials        696      695    -1     

Impacted Files	Coverage Δ	Complexity Δ
...main/java/com/networknt/consul/ConsulRegistry.java	50.00% <25.00%> (-1.24%)	21.00 <3.00> (ø)
...n/java/com/networknt/client/oauth/OauthHelper.java	36.93% <0.00%> (+0.60%)	28.00% <0.00%> (+2.00%)

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[stevehu]

The fix looks good. For active lookup triggered by request, always use 0L to get an immediate return. For the subscription thread, use block query by passing in the last index. In case that the last index returned from the Consul is smaller, reset the index to 0L.