ReplicatedPG:start_flush send a second delete

Suppose we start with the following in the cache pool:

30:[29,21,20,15,10,4]:[22(21), 15(15,10), 4(4)]+head

The object doesn't exist at 29 or 20.

First, we flush 4 leaving the backing pool with:

3:[]+head

Then, we begin to flush 15 with a delete with snapc 4:[4] leaving the
backing pool with:

4:[4]:[4(4)]

Then, we finish flushing 15 with snapc 9:[4] with leaving the backing
pool with:

9:[4]:[4(4)]+head

Next, snaps 10 and 15 are removed causing clone 10 to be removed leaving
the cache with:

30:[29,21,20,4]:[22(21),4(4)]+head

We next begin to flush 22 by sending a delete with snapc 4(4) since
prev_snapc is 4 <---------- here is the bug

The backing pool ignores this request since 4 < 9 (ORDERSNAP) leaving it
with:

9:[4]:[4(4)]

Then, we complete flushing 22 with snapc 19:[4] leaving the backing pool
with:

19:[4]:[4(4)]+head

Then, we begin to flush head by deleting with snapc 22:[21,20,4] leaving
the backing pool with:

22[21,20,4]:[22(21,20), 4(4)]

Finally, we flush head leaving the backing pool with:

30:[29,21,20,4]:[22(21*,20*),4(4)]+head

When we go to flush clone 22, all we know is that 22 is dirty, has snaps
[21], and 4 is clean. As part of flushing 22, we need to do two things:
1) Ensure that the current head is cloned as cloneid 4 with snaps [4] by
sending a delete at snapc 4:[4].
2) Flush the data at snap sequence < 21 by sending a copyfrom with snapc
20:[20,4].

Unfortunately, it is possible that 1, 1&2, or 1 and part of the flush
process for some other now non-existent clone have already been
performed.  Because of that, between 1) and 2), we need to send
a second delete ensuring that the object does not exist at 20.

Fixes: #9054
Backport: firefly
Signed-off-by: Samuel Just <sam.just@inktank.com>