Currently, the buffer builder tries to be friendly with the system allocators and ask for powers of two allocations: https://github.com/10gen/mongo/blob/34bb8ccdba458e000b7b0ce7b6c0c157619fcdd6/src/mongo/bson/util/builder.h#L460-L462

However, then later down the function call chains, this power of two size is increased by a few bytes: https://github.com/10gen/mongo/blob/fcdc972cd6f64bfe710161fb3a0ffc4a5ca329d1/src/mongo/util/shared_buffer.h#L73

In this issue, we want to fix this behaviour and make sure that the end allocated size asked from the system allocator (https://github.com/10gen/mongo/blob/47b380f03e8898f4706ff01fa2be64dfb72e0dba/src/mongo/util/allocator.cpp#L47) is a power of two.