add in IncrByAutoTs and DecrByAutoTs functions with tests

[maguec]

Similar to AddAutoTs we need to Incr and Decr counters without timestamps from threaded applications

[codecov]

Codecov Report

Merging #88 (a11d937) into master (8a44943) will decrease coverage by 0.30%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   77.88%   77.57%   -0.31%     
==========================================
  Files           7        7              
  Lines         434      446      +12     
==========================================
+ Hits          338      346       +8     
- Misses         55       57       +2     
- Partials       41       43       +2     

Impacted Files	Coverage Δ
client.go	82.24% <66.66%> (-1.20%)	⬇️

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[danni-m]

LGTM