osd: clear pg REPLAY when we reinitiate peering

osd: clear pg REPLAY when we reinitiate peering

Among other things, a stray/bad REPLAY flag can trigger a false activation
like so:

osd/PG.cc: In function 'void PG::activate(ObjectStore::Transaction&, std::list<Context*, std::allocator<Context*> >&, std::map<int, MOSDPGInfo*, std::less<int>, std::allocator<std::pair<const int, MOSDPGInfo*> > >*)':
osd/PG.cc:1641: FAILED assert(peer_info.count(peer))
 1: (PG::activate(ObjectStore::Transaction&, std::list<Context*, std::allocator<Context*> >&, std::map<int, MOSDPGInfo*, std::less<int>, std::allocator<std::pair<int const, MOSDPGInfo*> > >*)+0x817) [0x75e637]
 2: (OSD::activate_pg(pg_t, utime_t)+0x136) [0x6bfe86]
 3: (OSD::check_replay_queue()+0x134) [0x6c0074]
 4: (OSD::tick()+0x1db) [0x6d96eb]
 5: (OSD::C_Tick::finish(int)+0x1c) [0x74844e]
 6: (SafeTimer::EventWrapper::finish(int)+0x6d) [0x7fa2cf]
 7: (Timer::timer_entry()+0x454) [0x7fb52a]
 8: (Timer::TimerThread::entry()+0x19) [0x647c09]
 9: (Thread::_entry_func(void*)+0x20) [0x659f18]
 10: /lib/libpthread.so.0 [0x7fceb73d473a]
 11: (clone()+0x6d) [0x7fceb65fe69d]