Support mmap/sbrk interception for avoiding data copies between runtime and client

[lukemartinlogan]

Currently, we copy data between clients and runtime since the runtime is in a different address space. However, if we intercept mmap/sbrk and put all data in SHM, this will allow applications to avoid the data copy, particularly for GET.

Each process will have to register its private allocator with the runtime. The runtime will have to store a mapping between process ID and allocator.

A well-formed version will have to have a wreaper thread to unregister dead processes automatically.