ReplicatedPG: fix snapdir trimming

The previous logic was both complicated and not correct.  Consequently,
we have been tending to drop snapcollection links in some cases.  This
has resulted in clones incorrectly not being trimmed.  This patch
replaces the logic with something less efficient but hopefully a bit
clearer.

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>