Move all of knowledge about tx filtering/selection to the indexes implementation

[jansegre]

Previous discussion: https://github.com/HathorNetwork/hathor-core/pull/392#discussion_r882970149

In summary: hathor.indexes.IndexesManager is the de-factor filter for which type of transaction an index will be fed. This isn't ideal and that filtering should be up to the indexes themselves. For some indexes, like tips and timestamp, it will make sense for this selection to be configurable so it can be instantiated multiple times with different configurations (like only-blocks, only-txs, all).