Preserve the relative order in which property based routes are defined

[ukayani]

Addresses #1660

• this would ensure that route definitions from properties do not lose their relative ordering
• only affects routes that end up having the same order, such as ones typically defined via properties (default order = 0)
• using flatMap does not guarantee to preserve the order of elements, hence a switch to flatMapSequential to preserve order before the AnnotationAwareOrderComparator is used for sorting.

This would obviously have some overhead for needing to buffer elements to ensure ordering guarantees but probably isn't a big issue since there wouldn't be an extremely large amount of routes per locator.

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[codecov]

Codecov Report

Merging #1661 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1661   +/-   ##
=========================================
  Coverage     78.03%   78.04%           
  Complexity     1217     1217           
=========================================
  Files           156      156           
  Lines          4940     4941    +1     
  Branches        397      397           
=========================================
+ Hits           3855     3856    +1     
  Misses          879      879           
  Partials        206      206           

Impacted Files	Coverage Δ	Complexity Δ
...gateway/route/CompositeRouteDefinitionLocator.java	95.00% <100.00%> (+0.26%)	8.00 <1.00> (ø)
...ork/cloud/gateway/route/CompositeRouteLocator.java	100.00% <100.00%> (ø)	2.00 <1.00> (ø)

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[spencergibb]

Cherry-picked into 2.2.x via 73b01f8