Skip unnecessary file reading at startup

[nickdowell]

Goal

Improve notifier start time by doing less work.

Changeset

Defers reading config.json, metadata.json and state.json until building an OOM or Thermal Kill event (if detected) rather than reading them in -[BugsnagClient initWithConfiguration:].

Testing

Verified by running E2E tests locally - in particular features/barebone_tests.feature:190 and features/out_of_memory.feature:11 that ensure various items are correctly set in the payload, and running a full E2E on iOS 10 on CI.

[github-actions]

Bugsnag.framework binary size decreased by 2,280 bytes from 854,936 to 852,656 🎉

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  [ = ]       0   +17% +2.23Ki    [__LINKEDIT]
  +4.2%    +710  +4.2%    +710    [__TEXT]
   +22%    +576  +6.9%    +576    [__DATA]
  +3.6%    +140  +3.6%    +140    __TEXT,__gcc_except_tab
  -0.4%      -8  -0.4%      -8    Function Start Addresses
  -0.7%     -16  -0.7%     -16    Rebase Info
  -2.2%     -24  -2.2%     -24    __DATA,__objc_ivar
  -0.7%     -48  -0.7%     -48    __DATA,__objc_selrefs
  -0.1%    -236  -0.1%    -236    __TEXT,__text
  -0.9%    -504  -0.9%    -504    __DATA,__objc_const
  -2.0%    -614  -2.0%    -614    __TEXT,__objc_methname
  -0.6%    -912  -0.6%    -912    Symbol Table
  -0.7% -1.31Ki  -0.7% -1.31Ki    String Table
  -0.3% -2.23Ki  [ = ]       0    TOTAL

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