We store all pins in a single massive object so adding and removing pins is really slow when we have many pins.

This affects:

• ipfs dag add --pin=true
• ipfs add
• ipfs pin add
• ipfs pin rm

Listing pins also appears to be slow but for a different reason:

1. ipfs pin ls buffers pins in memory before sending them back to the user (see pin ls should stream the result #6304).
2. ipfs pin ls lists all pinned blocks, directly or indirectly, by default. Calling ipfs pin ls --type=recursive is much faster.

Proposed solutions:

• Use MFS for all pins: Store pins in MFS #4675
• Named pins: Named pins & pins stored in datastore #4757, Moving away from current pin system #4763

I'm filing this issue so we can have a single issue that succinctly describes the entire issue and all variants.