trim pmem values

Change-Id: I1bb3e81ad4c01795f40265d9010cb0170b22aa26
readahead: make context readahead more conservative

Try to prevent negatively impact moderately dense random reads on SSD.

Transaction-Per-Second numbers provided by Taobao:

		QPS	case
		-------------------------------------------------------
		7536	disable context readahead totally
w/ patch:	7129	slower size rampup and start RA on the 3rd read
		6717	slower size rampup
w/o patch:	5581	unmodified context readahead
Before, readahead will be started whenever reading page N+1 when it
happen to read N recently. After patch, we'll only start readahead
when *three* random reads happen to access pages N, N+1, N+2. The
probability of this happening is extremely low for pure random reads,
unless they are very dense, which actually deserves some readahead.

Also start with a smaller readahead window. The impact to interleaved
sequential reads should be small, because for a long run stream, the
the small readahead window rampup phase is negletable.

The context readahead actually benefits clustered random reads on HDD
whose seek cost is pretty high.  However as SSD is increasingly used for
random read workloads it's better for the context readahead to
concentrate on interleaved sequential reads.

Tested-by: Tao Ma <tm@tao.ma>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com
Post for review first, will include this in the next readahead series.