Added applyEntry function to Index.

[CSDUMMI]

I implemented this function because I want it to be easier to customize validation
of entries without modifying or knowing about the internal data structure used
in the KVStore.

If this PR is merged I plan on implementing similar functions on the DocStore, FeedStore and EventLog.

I think it is important for OrbitDB to be easily customizable and this feature should
lower the effort for customizing validation significantly.

In it's current form, the applyEntry function is not a breaking change, but an added
pattern that can be used to ease the customization efforts of Stores.

If this pattern is implemented I also feel ready to finally finish Part 5 of the Field Manual by introducing this pattern and implementing moderation rules using it.

Thanks for your consideration!

Custom Validation of Entries on the KeyValueIndex

To create a custom validation of the Index using the applyEntry function:

const KeyValueIndex = require("orbit-db-kvstore/src/KeyValueIndex")

// An index that only allows a key to be added iff it hasn't been added before.
class Index extends KeyValueIndex {
      applyEntry(entry) {
           if(entry.payload.op === "PUT" && this._index[entry.payload.key] === undefined) {
                super().applyEntry(entry) // or: this._index[entry.payload.key] = entry.payload.value
           }
      }
}