Fix a bug in castBoolImpl for arrays

Summary: Pretty bad bug.  It was ignoring the 'negate' arg.

Reviewed By: @bertmaher, @swtaarrs

Differential Revision: D1580833
Remove unused m_shouldCache bool in APCHandle

Summary: Nothing reads it, except to set it elsewhere.

Reviewed By: @alexmalyshev

Differential Revision: D1571331
Tweak headers so changing concurrent-shared-store.h doesn't rebuild world

Reviewed By: @alexmalyshev

Differential Revision: D1573497
Some clean up in APCHandle

Summary: - Move DataWalker to its own file

    - Add apc-typed-value.cpp and move some member function definitions there

    - Remove some unused or redundant functions

    - Normalize some naming (getFoo vs isFoo)

    - Renamed isObj (it really meant isNotSerializedObj) and inverted it so it
      parallels isSerializedArray.

    - Fix data race in setObjAttempted

    - Document thread-safety rules and add some consts accordingly

Reviewed By: @alexmalyshev

Differential Revision: D1574091