Evaluate memory overhead of icinga::Locked<...> in object attributes

As a countermeasure for race conditions, #9364 added a mutex for every object attribute with a type that's incompatible with std::atomic. At the moment, that's implemented using a dedicated std::mutex for every attribute for every single object. On my machine, sizeof(std::mutex) = 40, and if I compare the sizeof(icinga::Host) with and without these mutexes, that's a 70% increase. However, that won't result in a 70% increase in memory usage of Icinga 2 as a whole (for example, all strings like object names are dynamically allocated and thus not part of icinga::Host itself and aren't affected by this increase.

Tasks

1. Figure out how much of an effect this has on the total memory use of Icinga 2.

2. Improve this. One idea would be to take some inspiration from how something like atomic_load(const std::shared_ptr<T>*) is/can be implemented:

   These functions are typically implemented using mutexes, stored in a global hash table where the pointer value is used as the key.

   Note that if using only part of the address as the key, i.e. sharing the mutex between objects, this would reduce the memory requirements.