Updated to use @Async instead of CompletableFuture

[mrm9084]

Description

Fixes an issue when App Configuration is used in Java 11 in docker container with greater than 2 CPUs Refresh stops working.

This switches the CompletableFuture to @async which doesn't run into this issue.

[codecov-commenter]

Codecov Report

Merging #704 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #704   +/-   ##
=========================================
  Coverage     55.94%   55.94%           
  Complexity      757      757           
=========================================
  Files           181      181           
  Lines          3207     3207           
  Branches        237      237           
=========================================
  Hits           1794     1794           
  Misses         1277     1277           
  Partials        136      136           

Impacted Files	Coverage Δ	Complexity Δ
...loud/config/AppConfigurationAutoConfiguration.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...e/spring/cloud/config/AppConfigurationRefresh.java	88.88% <100.00%> (ø)	17.00 <1.00> (ø)

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[drago-draganov]

looks good!

[MSGaryWang]