Add doc/dev/LRU.md with per-loop LRU description

Several compilation units now use per-loop LRU lists, add basic
developers documentation on the design.

doc/dev/LRU.md

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+
+# Per-Loop LRU cleaning
+
+Several compilation units now employ per-loop LRU lists. When combined
+with other algorithms, this design allows LRU lists to be lock-free.
+
+When a new entry is created, it is assigned to the currently-running loop
+(`isc_tid()`), added to the loop's LRU list, and added to a global
+lock-free (`cds_lfht`) hash table.  Deletion of the entry by any loop will
+first delete it from the hash table, then schedule it to be removed from
+the LRU list by the entry's loop. If LRU cleaning happens in the meantime,
+the entry is processed normally.
+
+The badcache and unreachable primaries list are very simple LRUs that don't
+update the position of the entry in the list on cache hit; they just
+remove the old entry and insert new one.
+
+The ADB combines per-loop LRU lists with the SIEVE algorithm. On a
+cache hit, SIEVE marks the entry as "visited". There is no need to
+update the LRU list, so an off-loop cache hit is also lock-free.