`ipfs pin add` on large trees is extremely slow even when most of the content is already pinned

#### Version information:
go-ipfs version: 0.4.13-
Repo version: 6
System version: amd64/linux
Golang version: go1.9.4

#### Type:
Enhancement

#### Description:
`ipfs pin add` is slow on large data sets, because it needs to walk the entire tree in order to verify that it exists on the node (and, if not, fetch it from elsewhere) before the pin is considered successful. This is expected.

However, it looks like when pinning something *that itself contains other pinned objects*, it re-checks all of the contents, even though it should know that the contents are already present.

Reproduction:

hash=$(ipfs add )
ipfs pin ls --type=recursive # should output hash
ipfs files mkdir /test
ipfs files cp /ipfs/$hash /test/foo
ipfs pin add $(ipfs files stat --hash /test)

Ideally, `pin add` -- or whatever underlying function it calls to make sure the data is all available in the node -- should be smart enough to realize that when it encounters a recursively pinned object, it doesn't need to traverse that object's children, as they are guaranteed to be present.