[query] Improve readerIterator performance

[arnikola]

What this PR does / why we need it:
Takes logic from the Current() function used to calculate the values
into the Next() function so that it will only be executed once.

Below benchmarks from running the ./bin/read_data_files tool:

Before:

Running time: 732.597858ms
35238 series read
(48100.06 series/second)
2535668 datapoints decoded
(3461200.40 datapoints/second)

After:

Running time: 649.265258ms
35238 series read
(54273.66 series/second)
2535668 datapoints decoded
(3905442.30 datapoints/second)

[robskillington]

LGTM

[codecov]

Codecov Report

Merging #3512 (75c764f) into master (de6d212) will increase coverage by 0.1%.
The diff coverage is 57.1%.

@@           Coverage Diff            @@
##           master   #3512     +/-   ##
========================================
+ Coverage    55.9%   56.1%   +0.1%     
========================================
  Files         548     548             
  Lines       61373   61372      -1     
========================================
+ Hits        34328   34441    +113     
+ Misses      23937   23834    -103     
+ Partials     3108    3097     -11     

Flag	Coverage Δ
aggregator	57.3% <ø> (-0.1%)	⬇️
cluster	∅ <ø> (∅)
collector	54.3% <ø> (ø)
dbnode	60.6% <57.1%> (+0.2%)	⬆️
m3em	46.4% <ø> (ø)
metrics	19.8% <ø> (ø)
msg	74.6% <ø> (+<0.1%)	⬆️

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