Only support one instance of Ralloc in an application

[qtcwt]

Historically, Ralloc keeps regular parameter list of malloc and free for both C and C++. However, this makes it hard to support more than one instance of Ralloc in a single application. The support might be useful for persistent applications since they may have multiple independent persistent heaps for different usage (e.g., for different data structures and different back-end such as Montage).

As a result, in the short future, we want to support multiple instances of Ralloc while keeping the legacy interface as is.