Mas d34 leveled.i459 partialmerge (#460)

* Add test to replicate issue 459

Nothing actually crashes due to the issue - but looking at the logs there is the polarised stats associated with the issue.  When  merging into L3, you would normally expect to merge into 4 files - but actually we see FileCounter occasionally spiking.

* Add partial merge support

There is a `max_mergebelow` size which can be a positive integer, or infinity.  It defaults to 32.

If a merge from Level N covers less than `max_mergebelow` files in level N + 1 - the merge will proceesd as before.  If it has >= `max_mergebelow`, the merge will be curtailed when `max_mergebelow div 2` files have been created at that level.  The remainder for Level N will then be written, as well as for Level N + 1 up to the next whole file that has no yet been touched by the merge.

The backlog that prompted the merge will still exist - as the files in Level N have not been changed.  However, it is likely the next file picked will not be the same one, and will in probability have a lower number of files to merge (as the average is =< 8).

This will stop progress from being halted by long merge jobs, as they will exit out in a safe way after partial completion.  In the case where the majority of files covered  do not require a merge, then those files will be skipped the next time the remainder file is picked up for merge at Level N

test/end_to_end/testutil.erl

@@ -883,7 +883,11 @@ put_indexed_objects(Book, Bucket, Count, V) ->
     KSpecL =
         lists:map(
             fun({_RN, Obj, Spc}) ->
-                book_riakput(Book, Obj, Spc),
+                R = book_riakput(Book,Obj, Spc),
+                case R of
+                    ok -> ok;
+                    pause -> timer:sleep(?SLOWOFFER_DELAY)
+                end,
                 {testutil:get_key(Obj), Spc}
             end,
             ObjL1),