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.

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)
Merge pull request #10624 from jackpoz/instancesavemgr

Core/Instances: Fix mutex released after being deleted