MB-19995: Set default IO read buffer unit size to 4KB and its count to 16
MB-19995: Set default IO read buffer unit size to 4KB and its count to 16

* Decreasing the unit size improves the random read performance in heavy
DGM situation, by reducing the overhead of reading adjacent data.

* Observed more than 10% improvement on both XFS and Ext4 (block size 4KB),
on Samsung 850 Pro SSD.
(measured using https://github.com/couchbaselabs/ForestDB-Benchmark ,
 512 bytes * 64M docs, total 38GB data set size, compression disabled,
 7GB available RAM, 4 reader threads)

* No visible difference was seen on spinning disk under heavy DGM and
uniform random workloads, since disk seek time is the most critical
bottleneck so that other factors have relatively marginal impact. However,
when workloads have locality, 4KB unit size shows slightly (~5%) better
performance than 8KB even on spinning disk.

* To do: will upload a separate commit that makes both unit size and buffer
count configurable, in order to configure different setting for environments
that 4KB buffer doesn't work efficiently.

Change-Id: I40d3158aeaf8e17135cc5e76200e95ef1627c25d
Reviewed-on: http://review.couchbase.org/73259
Tested-by: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Trond Norbye <60edd2ef23891a753f231b0c6f161dc634079a93@gmail.com>
Reviewed-by: Dave Rigby <a09264da4832c7ff1d3bf1608a19f4b870f93750@couchbase.com>

couchbaselabs/couchstore,couchbaselabs/couchstore,couchbaselabs/couchstore