osd: New options for configuring new EC

osd: New options for configuring new EC

Adding three new configuration options which will apply once new EC
is in place:

osd_pool_default_flag_ec_optimizations

This allows EC optimizations to be turned on by default.

ec_extent_cache_size

This allows the user to specify the size of the per-shard extent cache if
they feel that the default 10MiB is too large or too small.

The default value may well change following more extensive testing.

ec_pd_write_mode

This is a development flag for testing the parity delta write RMW mechanism
within the EC code.  Setting to anything other than 0 will cause performance
problems.  It is provided as a test mechanism for performance and
teuthology.  Performance may wish too turn off all PDW writes for a particular
IO pattern. This will allow us to determine if the automatic mode should be
using conventional RMW writes.  The force-on mode allows testing on more
unusual scenarios and on smaller configurations.

Finally, we tweak the way optimisations are enabled, so as to be common between
enabling and default-enabled.

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