WIP: route discards via the I/O queue

[pwrobelse]

This set of changes is an another attempt to route discard requests via the I/O queue to ensure, that they are issued to the drive in a controlled way.

This PR:

• adds discard type to internal::io_request
• renames io_desc_read_write to queued_io_request_completion -- discards will use it too
• adds discard_to_write_ratio to io-properties.yaml
• adds discard direction to io_direction_and_length
• routes discard requests via the I/O queue
• introduces remove_file_via_blocks_discarding() function that can be used to remove files block by block