osd/ReplicatedPG: allow cache-evict on snaps

We do three things here:

 - make cache-evict a CACHE instead of WR op, allowing us to submit it
   on snaps (not just head)
 - allow eviction of a snap
 - verify that all snaps are missing before evicting a head

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: make cache-[try-]flush CACHE instead of WR ops

This will allow us to send a flush op on a snap.

Signed-off-by: Sage Weil <sage@inktank.com>
vstart.sh: allow MDS=0

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: refuse to flush when older dirty clones are present

If the next oldest clone is dirty, we cannot flush.  That is, we must
always flush starting with the oldest dirty clone.

Note that we can never have a sequence like dirty -> clean -> dirty,
because clones are only dirty on creation, are created in order, and cannot
be flushed (cleaned) out of order.  Thus checking the previous clone is
sufficient (and thankfully cheap).

Signed-off-by: Sage Weil <sage@inktank.com>
osd: add pg_log_entry_t event type CLEAN

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: construct appropriate snapc for flush/writeback

Construct a snap context that will trigger the appropriate cloning (if any)
on the base pool.

Signed-off-by: Sage Weil <sage@inktank.com>
ceph_test_rados_api_tier: test flush on snaps/clones

Signed-off-by: Sage Weil <sage@inktank.com>