osd: EC optimizations: additional types for EC

osd: EC optimizations: additional types for EC

Add some extra types required by the EC optimizations code:

raw_shard_id_t is an equivalent type to shard_id_t but is used
for storing raw shards. Strong typing prevents bugs where code
forgets to translate between the two types.

shard_id_map is a mini_flat_map indexed by shard_id_t which will
be used by the EC optimizations I/O path to track updates to
each shard.

shard_id_set is a bitset_set of shard_id_t which is a compact
and fast way of storing a set of shards involved in an EC
operation.

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