[PLAT-8750] Increase maxBreadcrumbs

[nickdowell]

Goal

Allow more breadcrumbs now that there is trimming in place to mitigate oversized payloads.

Changeset

Increases default maxBreadcrumbs from 50 to 100, and maximum allowed from 100 to 500.

Deletes breadcrumbs on a background queue to prevent increased number of breadcrumbs from slowing execution time of Bugsnag.start().

Testing

Amends existing unit test cases for changed default and limit.

Passes existing stress test.

[github-actions]

Bugsnag.framework binary size decreased by 40 bytes from 771,640 to 771,600 🎉

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.9%    +119  +0.9%    +119    [__TEXT]
  +0.6%     +64  +3.5%     +64    [__DATA]
  [ = ]       0  +0.3%     +40    [__LINKEDIT]
  -0.0%      -8  -0.0%      -8    String Table
  -0.1%      -8  -0.1%      -8    __DATA,__objc_selrefs
  -0.3%     -12  -0.3%     -12    __TEXT,__gcc_except_tab
  -0.0%     -12  -0.0%     -12    __TEXT,__text
  -0.4%     -12  -0.4%     -12    __TEXT,__unwind_info
  -0.1%     -22  -0.1%     -22    __TEXT,__objc_methname
  -0.0%     -24  -0.0%     -24    __DATA,__objc_const
  -0.0%     -32  -0.0%     -32    Symbol Table
  -0.2%     -32  -0.2%     -32    __DATA,__cfstring
  -0.3%     -61  -0.3%     -61    __TEXT,__cstring
  -0.0%     -40  [ = ]       0    TOTAL

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