quick-read, md-cache: selectively cache xattr data to conserve memory

The md-cache translator can cache items for a long period of time and
thus is sensitive to memory usage associated with the items it
references. This implements two changes to help conserve memory:

- quick-read - Migrate the file content data to a local dict and
	delete from the xdata dict that passes through the stack.
- md-cache - Create a local dict to selectively store the xattr
	data md-cache is interested in. This includes a slight
	optimization to not allocate an empty dict in the case
	where we have not received any xattr's of interest.

I've tested both changes independently and together by running a
readdirp test against several compiled source trees (~340k files).
The base test results in a 7.7GB RSS on the client. The quick-read
modification cuts RSS down to 4.1GB, which is still large due
to md-cache unintelligently caching a large number of empty
dictionaries. The combined modification cuts RSS down to 462MB
on the same workload.

NOTE: Theoretically the md-cache change should supercede the
quick-read change, but practically I save an extra 150MB or so with
both. I already had the change and consider it an improvement that
quick-read clean up after itself.

BUG: 812876
Change-Id: Id59734d12dd6476b0e32480939e633448adb6884
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-on: http://review.gluster.com/3268
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra G <raghavendra@gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>

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