Support for DROP INDEX

[federico-razzoli]

What would you like to be added or enhanced

DROP INDEX <table> (<column>)

Why is this needed / Additional context

When a regular database grows too much, you can historicise some data: move them to some archive, and then delete them from the main database. With an immutable db, we can't do that, so it is important to avoid writing unnecessary data.

An index may be created by mistake, or could become unnecessary at some point. In that case it would be desirable to drop it and:

1. If possible, the disk space it currently takes should be re-used in the future. But I understand that this could be against the immutable approach.
2. Even if the index is not physically dropped, no entries should be added in the future.
3. If you agree with point 2, if the index is re-created in the future, it could be possible to re-use the entries previously written on disk (at least for append-only indexes, eg timestamps).

[mmeloni]

Hey @federico-razzoli . Since indexes are stored in an immutable store I think that 1 is not feasible at the moment. 2 and 3 should be possible, since indexes are wrote in an inner keyvalue store.