Add and drop indexes online, without restart of the server

It should be possible, without restart of the server or downtime, to add or drop a secondary index.

The task should be split in two parts:
- part 1, removal of an index
- part 2, addition of a new index.

Once this is done, we may also want to explore these options:
- change a unique index to non-unique
- drop the primary key index (if there is another index, defined on the same set of fields, it becomes primary).