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

[futpib]

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

[Stebalien]

We can actually do even better and just ignore any files whose paths are tracked by the filestore (as changing those files will break everything anyways). The tricky part is the interaction with the add command itself.