addworker notification for preventing goroutines leak

[manc88]

Hi, came across the fact that many of our applications have started to notice goroutine leaks. Found out that
the reason is that we initialize the HTTPSyncTransport object many times in so-called handlers:

github.com/getsentry/sentry-go.(*HTTPTransport).worker(0xc0000000)

goroutine t.worker leaks in our case.
Using blocking HTTPSyncTransport is not beneficial for us and spoils latency.

You can take a look at the solution, thanks to which we managed to reduce the number of running goroutines by orders of magnitude!

Seems like it could be a solution for 731 issue

Thanks for your product anyway.

[codecov]

Codecov Report

Attention: Patch coverage is 68.75000% with 10 lines in your changes missing coverage. Please review.

Project coverage is 83.02%. Comparing base (6bd150f) to head (56934e2).

Files with missing lines	Patch %	Lines
transport.go	68.75%	8 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #894      +/-   ##
==========================================
+ Coverage   82.90%   83.02%   +0.12%     
==========================================
  Files          55       55              
  Lines        4627     5468     +841     
==========================================
+ Hits         3836     4540     +704     
- Misses        635      771     +136     
- Partials      156      157       +1     

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