Remove memory monitor and move translog operations threshold to shard level setting

[kimchy]

Now, with the improved memory management in 0.9 (hook into cleaning the cache when readers gets closed, and automatic setting of indexing buffer size), and the fixed bugs in Lucene for memory leaks, there is really no need for the memory cleaner.

The only other aspect that it was doing is flushing a shard based on the number of translog operations. This logic should really exists on the shard level and should be moved there.

The new setting for flush on translog operations is index.translog.flush_threshold, and defaults to 5000.

[kimchy]

Remove memory monitor and move translog operations threshold to shard level setting, closed by ee26d55.