Moved LRU cache implementation to separate files

[ZhilkinSerg]

Summary

SUMMARY: Infrastructure "Moved LRU cache implementation to separate files"```

Purpose of change

We would probably like to use LRU cache not only for map memory, but for other things too.

Describe the solution

Updated templates of LRU cache implementation, so it could be used with different key types and moved it to separate files.

[jbytheway]

The extern template lines should remain in map_memory.h.

[ZhilkinSerg]

What would be the benefit?

[jbytheway]

When you have explicit instantiations (as lru_cache.cpp has) you should have the extern template lines to declare them in a header. It tells the compiler "don't worry about instantiating this template; I've arranged for this to happen elsewhere".

Honestly, I was a bit surprised that this compiles at all, because of the implementation of lru_cache being not in the header. Looking into it a bit more, I see that extern template is a new feature in C++11 so I guess it is supposed to be able to work like this; the compiler's just trusting you to do the right thing.