use rcutils allocator in allocators.c to avoid direct use of malloc/free

[wjwwood]

This pull request makes sure that we're using rcutils_allocator_t everywhere and not using malloc, realloc, calloc, or free anywhere at all.

Connects to ros2/rcutils#102

[wjwwood]

I was a little worried about a performance impact with this change (as the rcutils allocator is across a shared library boundary most of the time), but I was looking at the decompiled code and apparently it jumps directly to __default_allocate in allocators.c of rcutils from rmw_allocate(). I think the compiler is smart enough to know that what rcutils_get_default_allocator() returns is stored locally as static and is unmodified, so it makes it const, and because rmw_allocate also doesn't modify the copy of it, the compiler just removes it all together and goes where .allocate points with what .state points to.

[dirk-thomas]

LGTM