ioctx: ListOmapValues & GetAllOmapValues added

[gierschv]

Added:

• ListOmapValues allows to iterate on all the key feteched during
  a read operation of an omap (the goal is to easily get the last
  read key from the C iterator. This one can then be used in another
  call as startAfter parameter).
• GetAllOmapValues returns all the keys / values of the whole omap
• GetOmapValues now uses ListOmapValues

[dotnwat]

I'd like to propose a slight change to the patch, but I'm open to anything :)

I think it is a good idea to strive for a 1-1 mapping with librados API. So, I'd propose that ideally we expose rados_create_read_op, rados_read_op_omap_get_vals, and rados_omap_get_next directly so that a user could construct something like ListOmapValues themselves. Having some convenience functions is also a good idea, so I think a common thing is just GetAllOmapValues() without any parameters, and users with extra needs can use the lower level API.

I don't mind merging this as it is now since the library is still evolving. Would that be useful now, and then start to expose the lower level stuff?

[gierschv]

Sure, I made this PR since I needed a lower level (i.e. knowing which is the last key read so I can create a new read operation to read the next keys), and I think the ListOmapValues I suggest here is a good start :)

[dotnwat]

Ok sounds good. If I get a chance soon I'll try to expose the rados op api directly and see how that looks.

[gierschv]

Thanks :)