When opening a level, enforce just enough merge

When re-opening a Hanoi data store, we need to
reestablish the invariant that there is always
room to inject a data file at the top level.

In a worst case scenario, every level has all of
A, B, and C; and thus needs to merge A+B -> X
fully in order to accommodate what the parent 
will inject. 2*BTREE_SIZE(Level) >= sizeof(A+B)

src/hanoi.erl

@@ -222,6 +222,8 @@ open_levels(Dir) ->
                      undefined,
                      lists:seq(MaxLevel, MinLevel, -1)),
 
+    ok = hanoi_level:incremental_merge(TopLevel, 2*?BTREE_SIZE(MaxLevel)),
+
     {ok, TopLevel}.
 
 parse_level(FileName) ->

src/hanoi_level.erl

@@ -59,7 +59,6 @@ open(Dir,Level,Next) when Level>0 ->
                                       process_flag(trap_exit,true),
                                       initialize(#state{dir=Dir,level=Level,next=Next})
                               end),
-    incremental_merge(PID, 2*?BTREE_SIZE(?TOP_LEVEL)),
     {ok, PID}.
 
 lookup(Ref, Key) ->