mon: OSDMonitor: Make 'osd pool rename' idempotent

'ceph osd pool rename' takes two arguments: source pool and dest pool.
If by chance 'source pool' does not exist and 'destination pool' does,
then, in order to assure it's idempotent, we want to assume that if
'source pool' no longer exists is because it was already renamed.

However, while we will return success in such case, we want to make sure
to let the user know that we made such assumption.  Mostly to warn the
user of such a thing in case of a mistake on the user's part (say, the
user didn't notice that the source pool didn't exist, while the dest did),
but also to make sure that the user is not surprised by the command
returning success if the user expected an ENOENT or EEXIST.

Fixes: #6635

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon/OSDMonitor: make racing dup pool rename behave

If we get dup pool rename requests that are racing, make sure the second
one comes back with 'success' if the rename entry already exists in the
pending_inc map.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #765 from ceph/wip-6635

mon: OSDMonitor: Make 'osd pool rename' idempotent

Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>