Use skpl as ledger_cache

Under assumption that it generates less GC noise (based on
micro-benchmark in leveled_tree eunit testing).

Note to confirm, needed to swap around the test order, and this showed
less collections in each position for skpl - and a < 10% performance hit

include/leveled.hrl

@@ -15,7 +15,7 @@
 %% Inker key type used for tombstones
 -define(INKT_TOMB, tomb).
 
--define(CACHE_TYPE, idxt).
+-define(CACHE_TYPE, skpl).
 
 -record(sft_options,
                         {wait = true :: boolean(),