[Discussion] Metadata features and implementation

There has been a lot of discussion and request for features that requires rclone to save metadata ( #977 discuss and references a bunch of other issues ). This issue is here to track discussion and progress for implementing both the metadata layer and the the relevant features.

For now I'm only going and try to implement two features which I hope should be relatively simple.
And since the main issue is implementing the metadata layer. it will make it a bit more trivial implementing other features.

Stuff needs discussion -

• How, where and which metadata is going to be saved #1336
• Keeping metadata Fresh and purging data of removed files #1335
• Handling collisions and discrepancies #1334

Features to implement -

• Two-way Data sync #118

Resume Uploads #1333 (There is an ACD issue #87 but I'm going to focus on OneDrive for now) Removed since this feature doesn't necessarily requires a database

Start by reading the discussion in #977 and the related issues #1336, #1335, and #1334 to understand the unresolved metadata design questions. Then review the two-way sync feature in #118; the issue is complete only when the metadata layer and an agreed implementation path for the listed feature are established.