'ipfs block rm' is slow

Issues:

* Running the command `ipfs block rm SomeBlock` is slow.
* While `ipfs block rm` is running, `ipfs repo gc`, `ipfs add`, and `ipfs pin add` hang.

Unfortunately, to make sure we're not removing a _pinned_ block, `ipfs block rm` currently:

1. Takes the _global_ GC lock as it's effectively a micro-GC. That is, we don't want a pin to _reference_ this block while we're in the middle of removing it.
2. Walks over all _pinned_ files to make sure the block isn't part of a pinned file.

Together this means that `ipfs block rm` effectively locks up much of the daemon for quite a while.

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The current recommendation is to avoid this command if at all possible.