OSDMonitor: add debug_fake_ec_pool

OSDMonitor: add debug_fake_ec_pool

This flag will cause ReplicatedPG to act as though the
pool were actually an EC pool in that operations will
be restricted to operations which can be locally rolled
back thereby allowing us to test the ReplicatedPG local
log rollback mechanisms independent of EC.  It will also
cause ReplicatedPG to use the async read mechanism on
the PGBackend implementation once it is implemented.

Signed-off-by: Samuel Just <sam.just@inktank.com>