airframe-rx: Add Rx.cache.getCurrent

[xerial]

I've found it's necessary to implement components that have some public methods referencing Rx data:

class Selector extends RxElement { 
   private val list = getList.map(x => ...).cache 

   def selected = list.getCurrent.map(lst => lst(selectedIndex))
}

[codecov]

Codecov Report

Merging #1572 (816eb64) into master (b46be2a) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1572      +/-   ##
==========================================
- Coverage   82.94%   82.94%   -0.01%     
==========================================
  Files         306      306              
  Lines       12146    12147       +1     
  Branches      740      740              
==========================================
  Hits        10075    10075              
- Misses       2071     2072       +1     

Impacted Files	Coverage Δ
...frame-rx/src/main/scala/wvlet/airframe/rx/Rx.scala	81.01% <100.00%> (+0.24%)	⬆️
...rx/src/main/scala/wvlet/airframe/rx/RxOption.scala	55.81% <100.00%> (+1.05%)	⬆️
...rx/src/main/scala/wvlet/airframe/rx/RxRunner.scala	91.92% <0.00%> (-0.45%)	⬇️
...shared/src/main/scala/wvlet/log/io/StopWatch.scala	80.00% <0.00%> (-0.21%)	⬇️

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[xerial]

cc: @shimamoto