rbd-replay-prep: simplify IO dependency calculation

rbd-replay-prep: simplify IO dependency calculation

Only track read-after-write and write-after-write IO dependencies
via the associated write completions.  All IO events after a write
completion are considered to be dependent and can be pruned down
to at most the number of concurrent IOs.  This reduces the prep
time from a simple 'rbd bench-write' from over 4 hrs down to seconds.

Fixes: #13378, #13384
Backport: hammer

Signed-off-by: Jason Dillaman <dillaman@redhat.com>