Improves multiple threads parsing efficiency

[qingzhongli]

Multiple threads concurrently parsing XML files causes most to be blocked. This patch changes the synchronized map to threadlocal map, which improves multiple threads parsing efficiency.

[codecov-io]

Codecov Report

Merging #57 into master will increase coverage by <.01%.
The diff coverage is 90%.

@@             Coverage Diff              @@
##             master      #57      +/-   ##
============================================
+ Coverage     39.93%   39.93%   +<.01%     
- Complexity     1493     1494       +1     
============================================
  Files           145      145              
  Lines          9490     9487       -3     
  Branches       1192     1192              
============================================
- Hits           3790     3789       -1     
+ Misses         5320     5319       -1     
+ Partials        380      379       -1

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/org/dom4j/tree/QNameCache.java	74.24% <90%> (+0.32%)	16 <4> (ø)	⬇️
src/main/java/org/dom4j/tree/NamespaceCache.java	90.69% <0%> (+2.32%)	13% <0%> (+1%)	⬆️

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