Logging: Fix extremely small line buffer size

In logStream.hpp, the _smallbuf member is 64 chars long. If I am not misunderstanding the code, this will create malloc-free calls for almost each and every LogStream. Would be nice to align this size to vwrite_buffer_size (512) in logTagSet.cpp.

Also update the bad comments regarding stack allocation (they are always stack allocated):
  // Helper class, maintains the line buffer. For small line lengths,
  // we avoid malloc and use a fixed sized member char array. If LogStream
  // is allocated on the stack, this means small lines are assembled
  // directly on the stack.

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  // Prevent operator new for LogStream.
  static void* operator new (size_t);
  static void* operator new[] (size_t);