tests: misc-concurrent-io: refactor code and add new tests

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
gdb: osv zfs: add zfs prefetch stats

Useful for tracking ZFS prefetch efficiency.

Signed-off-by: Raphael S. Carvalho <raphaelsc@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
cli: No print from formatting utilities

Formatting functions changed to format the output and return it instead of
printing it.

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
cli: Add API renderer for response class 'Threads'

Fixes #416.

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>