Add latest entries capability

[Asone]

Right now the core service keeps in memory the X last entries for each feed in memory.

It could be useful to allow to retrieval of the latest entries stored in memory, be it for a single feed or for the whole.

[Asone]

Current code does not allow to retrieve the latest entries in memory.

This is due to the fact we don't store the entries themselves, but rather the guid of an entry in a Vec to process the matching.

Thus, i still like the idea to be able to retrieve the latest entries, however i think i'd have to find a way to make this efficient. Replacing current map storage with a HashMap containing the whole entry would greatly increase the memory consumption, which seems to me undesirable.

Maybe we could use an optional flag to enable/disable a parallel storage that would allow us such retrieval.