optimize:optimize the magic num of date at UUIDGenerator

[lightClouds917]

Ⅰ. Describe what this PR did

optimize:optimize the magic num of date at UUIDGenerator

Ⅱ. Does this pull request fix one issue?

Ⅲ. Why don't you add test cases (unit test/integration test)?

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[codecov-io]

Codecov Report

Merging #2400 into develop will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##             develop    #2400      +/-   ##
=============================================
- Coverage      51.81%   51.80%   -0.01%     
+ Complexity      2698     2696       -2     
=============================================
  Files            517      517              
  Lines          16759    16761       +2     
  Branches        2028     2028              
=============================================
  Hits            8683     8683              
  Misses          7267     7267              
- Partials         809      811       +2     

Impacted Files	Coverage Δ	Complexity Δ
...r/src/main/java/io/seata/server/UUIDGenerator.java	65.62% <100.00%> (+2.29%)	5.00 <0.00> (ø)
...o/seata/server/coordinator/DefaultCoordinator.java	54.35% <0.00%> (-0.52%)	28.00% <0.00%> (-1.00%)
...in/java/io/seata/server/session/GlobalSession.java	84.00% <0.00%> (-0.45%)	72.00% <0.00%> (-1.00%)

[slievrly]

LGTM

[zjinlei]

LGTM