[PLAT-11456] Make sure all paths of weak span list are protected with a mutex

[kstenerud]

The count method of weak span list wasn't protected by a mutex, causing TSAN issues.

[github-actions]

BugsnagPerformance.framework binary size increased by 152 bytes from 489,664 to 489,816

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%     +80  +0.1%     +80    Symbol Table
  +0.1%     +76  +0.1%     +76    __TEXT,__text
  +0.0%     +72  +0.0%     +72    String Table
  +0.0%      +4  +0.0%      +4    __TEXT,__gcc_except_tab
  +0.1%      +4  +0.1%      +4    __TEXT,__unwind_info
  -0.3%     -84  -0.3%     -84    [__TEXT]
  [ = ]       0  -8.2%    -152    [__LINKEDIT]
  +0.0%    +152  [ = ]       0    TOTAL

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