This series adds a wrapper for the default rjson allocator which throws on allocation/reallocation failures. It's done to work around several rapidjson (the underlying JSON parsing library) bugs - in a few cases, malloc/realloc return value is not checked, which results in dereferencing a null pointer (or an arbitrary pointer computed as 0 + `size`, with the `size` parameter being provided by the user). The new allocator will throw an `rjson:error` if it fails to allocate or reallocate memory.
This series comes with unit tests which checks the new allocator behavior and also validates that an internal rapidjson structure which we indirectly rely upon (Stack) is not left in invalid state after throwing. The last part is verified by the fact that its destructor ran without errors.
Fixes#8521
Refs #8515
Tests:
* unit(release)
* YCSB: inserting data similar to the one mentioned in #8515 - 1.5MB objects clustered in partitions 30k objects in size - nothing crashed during various YCSB workloads, but nothing also crashed for me locally before this patch, so it's not 100% robust
relevant YCSB workload config for using 1.5MB objects:
```yaml
fieldcount=150
fieldlength=10000
```
Closes#8529
* github.com:scylladb/scylla:
test: add a test for rjson allocation
test: rename alternator_base64_test to alternator_unit_test
rjson: add a throwing allocator
c36549b22e