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The `SHAMapItem` class contains a variable-sized buffer that holds the serialized data associated with a particular item inside a `SHAMap`. Prior to this commit, the buffer for the serialized data was allocated separately. Coupled with the fact that most instances of `SHAMapItem` were wrapped around a `std::shared_ptr` meant that an instantiation might result in up to three separate memory allocations. This commit switches away from `std::shared_ptr` for `SHAMapItem` and uses `boost::intrusive_ptr` instead, allowing the reference count for an instance to live inside the instance itself. Coupled with using a slab-based allocator to optimize memory allocation for the most commonly sized buffers, the net result is significant memory savings. In testing, the reduction in memory usage hovers between 400MB and 650MB. Other scenarios might result in larger savings. In performance testing with NFTs, this commit reduces memory size by about 15% sustained over long duration. Commit 2 of 3 in #4218.