Introduce partitioned unordered maps:

This commit implements partitioned unordered maps and makes it possible
to traverse such a map in parallel, allowing for more efficient use of
CPU resources.

The `CachedSLEs`, `TaggedCache`, and `KeyCache` classes make use of the
new functionality, which should improve performance.

src/test/csf/Peer.h

@@ -919,7 +919,7 @@ struct Peer
     start()
     {
         // TODO: Expire validations less frequently?
-        validations.expire();
+        validations.expire(j);
         scheduler.in(parms().ledgerGRANULARITY, [&]() { timerEntry(); });
         startRound();
     }