osd: Introduce pool flag for "split IO" and Plugin flag for "direct read"

osd: Introduce pool flag for "split IO" and Plugin flag for "direct read"

These flags will currently behave as follows:

1. The pool flag is never set, unless by a user with the osd_pool_default_flags
   config option.
2. The pool flag will be removed for EC pools where the plugin does not support
   direct reads.
3. Replica pools will never remove the flag.

The intention is to eventually invert this logic and allow split IOs upon
upgrade to Umberella in this same function.

Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>