Core/Instances: Fix mutex released after being deleted

Modify how InstanceSave is deleted so the local mutex can be released before deleting the class itself. When removing a player or a group from the InstanceSave, InstanceSaveMgr flags the save to be deleted and then the InstanceSave checks the flag and deletes itself, only after the mutex has been released.
Feel free to modify the code in case there's a more elegant solution to the issue.

Valgrind log:
 Invalid read of size 4
  at 0x662662B: __pthread_mutex_unlock_usercnt (pthread_mutex_unlock.c:52)
  by 0x55D3C55: ACE_OS::mutex_unlock(pthread_mutex_t*) (OS_NS_Thread.cpp:2335)
  by 0xB20057: Player::CleanupsBeforeDelete(bool) (OS_NS_Thread.inl:3519)
  by 0xD0E2FA: WorldSession::LogoutPlayer(bool) (WorldSession.cpp:527)
  by 0xC66D34: WorldSession::HandleLogoutRequestOpcode(WorldPacket&) (MiscHandler.cpp:403)
  by 0xD0EA82: WorldSession::Update(unsigned int, PacketFilter&) (WorldSession.cpp:312)
  by 0xD9AD66: World::UpdateSessions(unsigned int) (World.cpp:2615)
  by 0xD9BEC4: World::Update(unsigned int) (World.cpp:1978)
  by 0xA035E5: WorldRunnable::run() (WorldRunnable.cpp:60)
  by 0xEC8D39: ACE_Based::Thread::ThreadTask(void*) (Threading.cpp:183)
  by 0x55D7555: ACE_OS_Thread_Adapter::invoke() (OS_Thread_Adapter.cpp:103)
  by 0x6622B4F: start_thread (pthread_create.c:304)
Address 0x1884bb08 is 56 bytes inside a block of size 104 free'd
  at 0x4C279DC: operator delete(void*) (vg_replace_malloc.c:457)
  by 0xC9D533: InstanceSaveManager::RemoveInstanceSave(unsigned int) (InstanceSaveMgr.cpp:159)
  by 0xC9E826: InstanceSave::UnloadIfEmpty() (InstanceSaveMgr.cpp:238)
  by 0xB2003E: Player::CleanupsBeforeDelete(bool) (InstanceSaveMgr.h:84)
  by 0xD0E2FA: WorldSession::LogoutPlayer(bool) (WorldSession.cpp:527)
  by 0xC66D34: WorldSession::HandleLogoutRequestOpcode(WorldPacket&) (MiscHandler.cpp:403)
  by 0xD0EA82: WorldSession::Update(unsigned int, PacketFilter&) (WorldSession.cpp:312)
  by 0xD9AD66: World::UpdateSessions(unsigned int) (World.cpp:2615)
  by 0xD9BEC4: World::Update(unsigned int) (World.cpp:1978)
  by 0xA035E5: WorldRunnable::run() (WorldRunnable.cpp:60)
  by 0xEC8D39: ACE_Based::Thread::ThreadTask(void*) (Threading.cpp:183)
  by 0x55D7555: ACE_OS_Thread_Adapter::invoke() (OS_Thread_Adapter.cpp:103)