Set the userId on the analytics executor thread.

[f2prateek]

Previously we were setting the userId on the thread that identify is called (which may be any thread), but reading the userId on a background thread.

Because of this, the userId on events called before identify may have the userId from "newer" events.

analytics.identify("foo");
analytics.track("event"); -> may have userId foo
analytics.identify("bar");

To fix this, we now also set the userId on the background thread.

Originally referenced in #522. Note that my original solution proposed we read the userId on the main thread to fix the issue, but the fix implemented here is to read the userId on the background thread instead.

Ref: LIB-120

Closes #522

[codecov-io]

Codecov Report

Merging #573 into master will not change coverage.
The diff coverage is 85.71%.

@@           Coverage Diff           @@
##           master     #573   +/-   ##
=======================================
  Coverage   82.94%   82.94%           
=======================================
  Files          30       30           
  Lines        2451     2451           
  Branches      281      281           
=======================================
  Hits         2033     2033           
  Misses        334      334           
  Partials       84       84

Impacted Files	Coverage Δ
...src/main/java/com/segment/analytics/Analytics.java	75.34% <85.71%> (ø)	⬆️

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[flagbug]

@f2prateek Is there a plan to ever release this fix in a stable version? I think we're hitting this issue, but since we've pinned the Segment library to stable versions, it looks like this has never been picked up in our app...