osd: clear newly_removed_snaps on osdmap update if unchanged

osd: clear newly_removed_snaps on osdmap update if unchanged

We only want to apply _newly_ removed snaps once, or else we try to trim
the same snaps multiple times, and crash like so

./include/interval_set.h: In function 'void interval_set<T>::insert(T, T) [with T = snapid_t]':
./include/interval_set.h:202: FAILED assert(0)
 1: (interval_set<snapid_t>::insert(snapid_t, snapid_t)+0x12c) [0x6b1728]
 2: (interval_set<snapid_t>::insert(snapid_t)+0x2f) [0x6b195d]
 3: (ReplicatedPG::snap_trimmer()+0x1c02) [0x66d5d6]
 4: (OSD::SnapTrimWQ::_process(PG*)+0x24) [0x6dc2ac]
 5: (ThreadPool::WorkQueue<PG>::_void_process(void*)+0x28) [0x6fa28a]
 6: (ThreadPool::worker()+0x23a) [0x7f57a4]
 7: (ThreadPool::WorkThread::entry()+0x19) [0x73e9b1]
 8: (Thread::_entry_func(void*)+0x20) [0x6508a4]
 9: /lib/libpthread.so.0 [0x7fa2707dc73a]
 10: (clone()+0x6d) [0x7fa26fa0669d]

Signed-off-by: Sage Weil <sage@newdream.net>