Add parallelism for file sorter chunk sorting

The file sorter reads 64M of records, sort it and
writes to disk upto 16 files and uses this intemediate
sort files for merge phase. This in-memory sort is
cpu bound (45 % time is spend in comparator).

Added ability to run two sorts jobs at a time
in parallel. The benchmark results of running an
initial index build with 10M documents and 3 views are
as follows (Mac/SSD):

Before:
real 8m14.328s
user 8m1.810s
sys  0m10.472s

After:
real 5m35.886s
user 8m37.032s
sys  0m11.566s

Change-Id: I13457ecb482c717533e92c99776a180854c98dc9
Reviewed-on: http://review.couchbase.org/32997
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>