Refactor Penciller Push

Two aspects of pushing to the penciller have been refactored:
1 - Allow the penciller to respond before the ETS table has been updated
to unlock the Bookie sooner.
2 - Change the way the copy of the memtable is stored to work more
effectively with snapshots wihtout locking the Penciller any further on
a snapshot or push request

include/leveled.hrl

@@ -30,6 +30,7 @@
 
 -record(penciller_options,
                         {root_path :: string(),
+                        penciller :: pid(),
                         max_inmemory_tablesize :: integer()}).
 
 -record(bookie_options,