osd: Restrict choice of primary shard for ec_optimizations pools

osd: Restrict choice of primary shard for ec_optimizations pools

Pools with ec_optimizations enabled have restrictions on which
shards are permitted to become the primary because not all shards
are updated for every I/O.

To preserve backwards compatibility with downlevel clients
pg_temp is used as the method to override the selection of
primary by OSDMap. Directly changing the logic in OSDMap
would have meant that all clients need to be upgraded to
tentacle before using optimized EC pools, so was discounted.
Using primary_temp to set the primary for an EC pool is
not reliable because under error conditions an OSD can store
multiple shards for the same PG and primary_temp cannot
define which of these shards will be choosen.

For optimized EC pools pg_temp is shuffled so that the
non-primary shards are listed last. This means that the
existing logic in OSDMap that picks the first available
shard as the primary will avoid selecting a non-primary
shard. OSDMonitor applies the shuffle when pg_temp is set,
this is then reverted in PeeringState when initializing the
acting set after OSDMap has selected the primary.

PeeringState::choose_acting is modified to set pg_temp if
OSDMap has selected a non-primary shard, this will cause
a new OSDMAP to be published which will persuade
OSDMap to select a primary shard instead.

Signed-off-by: Bill Scales <bill_scales@uk.ibm.com>