Add support for getting the alignment (stripe size) in bytes for pools that require/support it.
Fixes#739
This commit adds two new API calls to IOContext, one to know if the pool requires alignment and
the second one to get the stripe size in bytes:
RequiresAlignment() (bool, error)
Alignment() (uint64, error)
Signed-off-by: Daniel M. Lambea <dmlambea@tenerife.es>