[enco] Decouple buffer enumerator and memory allocator (#1941)

[enco] Decouple buffer enumerator and memory allocator (#1941)

The current implementation of intermediate buffer enumerator is a bit
complicated as it should exclude input/output bags due to the design of
host memory allocator.

With this commit, host memory allocator is responsible for input/output
bag management. This design allows us to reduce coupling between intermediate
buffer enumerator and host memory allocator.

Signed-off-by: Jonghyun Park <jh1302.park@samsung.com>