storage/localstore: fix for pinned chunks that are GC under certain conditions

[santicomp2014]

This PR is a fix, for pinned chunks that are being garbage collected when multiple GC is run.

This can be verified by

• uploading a chunk/pinning it
• downloading it (works ok)
• upload random chunks multiple times the store capacity to flush all chunks with GC
• try to download the chunk it can fail if the GC is run multiple times

This should be tested by pinning once the content, and also by pinning multiple times the same chunk and repeating the test.

After the first run of the Garbage collection, the pinned chunk is removed from gcExcludeIndex
So the next time the GC is run it will be deleted from the chunks.

I moved this delete from gcExcludeIndex from removeChunksInExcludeIndexFromGC (gc.go)
to setUnpin (mode_set.go)

Another issue was the metric localstore/gc/exclude/excluded-count this was only increasing, also decreased it in the setUnpin

closes #2195

[mortelli]

awaiting comment from @jmozah before properly reviewing this

[mortelli]

although not strictly a global-pinning issue, i have added the label since it impacts its implementation.

(keep in mind this merges to master)

[santicomp2014]

This PR was closed due to feedback with @jmozah
A new PR (#2200) is created with correct fix for local pinning bug.