fix(localstore): remove reserve calculation from localstore

[aloknerurkar]

Checklist

• I have read the coding guide.
• My change requires a documentation update, and I have done it.
• I have added tests to cover my changes.
• I have filled out the description and linked the related issues.

Description

During the recent storage incentives changes, we found that the reserve size calculation in the localstore was off. A bunch of efforts has already been made to understand and fix this, but due to the dynamic nature of storage depth, it seems like it is not possible to have a static count of reserve size stored in the localstore, which can represent the current reserve size.

The reserve size is now synonymous with the pullIndex as this is the index we use to generate the sample. This index will now be used to represent the reserve chunks in the localstore. So if chunks are put into GC index, which represents the cache, we should remove them from pullIndex as they are no longer part of the reserve. This change will make the addition of pullIndex more strict and only allow syncing related contexts to add the pullIndex.

As the reserve size calculation is now completely removed, there are a lot of simplifications in the mode_put file.

Reserve evictions will now be triggered if we are above reserve capacity as computed with the depth seen by the depthmonitor. This is periodic and will only happen if we are above capacity. Localstore will only trigger GC evictions from now.

All the unit tests are adjusted to now verify the new expected values in the indexes.

A bug related to postageIndex collisions is also fixed. During GC, we were not deleting the postageIndexIndex entry, which led to us searching for a chunk that is not present in the localstore. #3039

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This change is 

[aloknerurkar]

comment needs to be updated

[mrekucci]

The repetitive error handling can be improved be employing the following:

err = nil
deleteInBatch := func(fn func(*leveldb.Batch, shed.Item) error, batch *leveldb.Batch, keyFields shed.Item) {
	if err != nil {
		return
	}
	err = fn(batch, keyFields)
}

deleteInBatch(db.retrievalDataIndex.DeleteInBatch, batch, item)
deleteInBatch(db.retrievalAccessIndex.DeleteInBatch, batch, item)
deleteInBatch(db.pushIndex.DeleteInBatch, batch, storedItem)
deleteInBatch(db.pullIndex.DeleteInBatch, batch, item)
deleteInBatch(db.gcIndex.DeleteInBatch, batch, item)
deleteInBatch(db.postageIndexIndex.DeleteInBatch, batch, storedItem)
deleteInBatch(db.postageChunksIndex.DeleteInBatch, batch, item)

[aloknerurkar]

Interesting. This can be done in a lot of places actually. But I dont want to prolong this change as we need to start testing. Will do this in a follow-up PR later.