fix: Lock global options whenever they are accessed

[Swatinem]

Wraps all accesses of the global objects object in a lock. This also changes the DSN to be a heap allocated and refcounted type, since the http workers need to separately own a reference.

fixes #280

[codecov-commenter]

Codecov Report

Merging #333 into master will increase coverage by 0.29%.
The diff coverage is 78.98%.

@@            Coverage Diff             @@
##           master     #333      +/-   ##
==========================================
+ Coverage   87.37%   87.66%   +0.29%     
==========================================
  Files          49       49              
  Lines        4039     4045       +6     
==========================================
+ Hits         3529     3546      +17     
+ Misses        510      499      -11     

reworked everything

[Swatinem]

After discussing this a little bit, I completely reworked this again.
Instead of holding the lock for a long period of time, the locked section is now super small, just loading the global options and incrementing their refcount.
the SENTRY_WITH_OPTIONS macro now does that and decrements the refcount afterwards.

The only mutable access to options now is the user consent, which was changed to an atomic.

Instead of swapping out transports, capturing an event was split up into preparing the event, and then capturing it with an explicit transport. This also gets rid of the special case breakpad transport because the minidump can now be attached directly to an envelope instead of making roundtrips via that special transport.

While at it, inproc/breakpad now only send a single envelope on crash, with the session in it, which also now has an error count that covers the crash itself.