Generate timestamps at call site rather than asynchronously

[prayansh]

This PR implements generation of timestamps for payloads at call site rather than asynchronously, enabling us to generate more accurate timestamps for each payload. This feels like it should be a default behaviour and is thus not behind a feature flag.

This should not have a major performance impact, since we only add the generation of a timestamp to the main thread.

[codecov-commenter]

Codecov Report

Merging #686 into master will decrease coverage by 0.11%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #686      +/-   ##
==========================================
- Coverage   27.35%   27.24%   -0.12%     
==========================================
  Files          32       32              
  Lines        2716     2727      +11     
  Branches      318      318              
==========================================
  Hits          743      743              
- Misses       1918     1929      +11     
  Partials       55       55              

Impacted Files	Coverage Δ
...src/main/java/com/segment/analytics/Analytics.java	0.00% <0.00%> (ø)

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