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

include/leveled.hrl

@@ -33,6 +33,7 @@
 %%% Non-configurable startup defaults
 %%%============================================================================
 -define(MAX_SSTSLOTS, 256).
+-define(MAX_MERGEBELOW, 24).
 -define(LOADING_PAUSE, 1000).
 -define(LOADING_BATCH, 1000).
 -define(CACHE_SIZE_JITTER, 25).
@@ -109,7 +110,8 @@
                         press_level = ?COMPRESSION_LEVEL :: non_neg_integer(),
                         log_options = leveled_log:get_opts() 
                             :: leveled_log:log_options(),
-                        max_sstslots = ?MAX_SSTSLOTS :: pos_integer(),
+                        max_sstslots = ?MAX_SSTSLOTS :: pos_integer()|infinity,
+                        max_mergebelow = ?MAX_MERGEBELOW :: pos_integer()|infinity,
                         pagecache_level = ?SST_PAGECACHELEVEL_NOLOOKUP
                             :: pos_integer(),
                         monitor = {no_monitor, 0}