limit grooming activities to one thread.  Allow overlapped layers to submit smaller batches via grooming.  both changes are to space out write operations in normal times, giving Read / Iterator calls more disk access.
revert to original rule of forcing both level 0 and 1 to fadvise without regard to memory position.  Anya test craters without this.  When it has a low memory to vnode ratio, the re-reads of level 1 after a compaction kill performance for 90 minutes.
updated two tests.  changed magic number 4 to a descriptive constant kL0_GroomingTrigger that matches existing constants.
Merge pull request #140 from basho/mv-pacing

Give Read and Iterator calls more consistent disk access on moderately loaded systems.