[SharedCache] Fix some memory leaks in shared cache loading

[bdash]

This fixes three sources of leaks:

1. MMappedFileAccessor::ReadBuffer was returning a heap-allocated DataBuffer, but no callers were ever deleting it. There does not appear to be any reason to heap allocate the DataBuffer as the type is effectively a smart pointer wrapper around BNDataBuffer. It is now returned by value instead.
2. MMappedFileAccessor::ReadBuffer was allocating a buffer, copying data into it, and then handing that allocation to the DataBuffer constructor. The constructor copies data into a new allocation it owns so this allocation is unnecessary and was being leaked.
3. SharedCache::Store was appending a JSON array to itself rather than to a similarly named variable. This cyclic data structure was not correctly freed.

Before this change loading a macOS shared cache would leak over 1GB of memory:

leaks Report Version: 4.0, multi-line stacks
Process 34177: 30805047 nodes malloced for 11333150 KB
Process 34177: 2512405 leaks for 1087361648 total leaked bytes.

After this change the leaks are in the 10s of kilobytes and appear to be unrelated to shared cache loading:

leaks Report Version: 4.0, multi-line stacks
Process 69005: 25357627 nodes malloced for 9319529 KB
Process 69005: 240 leaks for 18400 total leaked bytes.

Additionally, the memory footprint of binaryninja after loading the macOS shared cache drops from 8.3GB to 6.9GB.

[CLAassistant]

All committers have signed the CLA.

[bdash]

@0cyn

[bdash]

Looks like these changes were cherry-picked into dev in 0d27f74 and d553396.