Make add faster by trusting file modification time (mtime), size and path

#### Version information:
```
go-ipfs version: 0.4.17-
Repo version: 7
System version: amd64/linux
Golang version: go1.10.3
```

#### Type:
Feature

#### Description:
When running `ipfs add --recursive --nocopy ./huge-dir` ipfs rehashes all files, even the ones already added.

This process could be faster with an option to skip already added files unless their mtime or size changed in the underlying filesystem.

Some mtime discussion already happened in #875