implementing new retry after protocol

[marandaneto]

there are a few changes that are not related to the new retry after protocol, but it's mostly to make the code nicer :) like eg Nullable/NonNull annotations and making fields immutable, you can ignore this one and focus on the other things.

[codecov-io]

Codecov Report

Merging #306 into feat/session will increase coverage by 0.98%.
The diff coverage is 47%.

@@                Coverage Diff                 @@
##             feat/session     #306      +/-   ##
==================================================
+ Coverage           53.73%   54.72%   +0.98%     
- Complexity            700      705       +5     
==================================================
  Files                  80       80              
  Lines                3530     3490      -40     
  Branches              324      331       +7     
==================================================
+ Hits                 1897     1910      +13     
+ Misses               1496     1443      -53     
  Partials              137      137

Impacted Files	Coverage Δ	Complexity Δ
...java/io/sentry/core/transport/TransportResult.java	90.9% <100%> (+5.19%)	2 <1> (ø)	⬇️
...e/src/main/java/io/sentry/core/EnvelopeSender.java	62.85% <100%> (ø)	12 <3> (ø)	⬇️
...java/io/sentry/core/transport/AsyncConnection.java	33.11% <13.79%> (-4.22%)	5 <1> (ø)
...c/main/java/io/sentry/core/cache/SessionCache.java	5.36% <3.33%> (-0.43%)	2 <1> (ø)
...try/core/transport/RetryingThreadPoolExecutor.java	78.94% <75%> (+24.1%)	7 <3> (-7)	⬇️
...n/java/io/sentry/core/transport/HttpTransport.java	67.76% <86.27%> (+31.46%)	18 <13> (+12)	⬆️

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