MINOR: peers: Add a LRU cache implementation for dictionaries.

MINOR: peers: Add a LRU cache implementation for dictionaries.

We want to send some stick-table data fields stored as strings in dictionaries
without consuming too much memory and CPU. To do so we implement with this patch
a cache for send/received dictionaries entries. These dictionary of strings entries are
stored in others real dictionary entries with an identifier as key (unsigned int)
and a pointer to the dictionary of strings entries as values.