Use gevent to parallelize storage operations in get_tags

This is a band-aid for #614

For context, hosted from a GCE instance with GCS being the backing
store, With this patch, we are able to 'docker pull' 859 tags without a
timeout.  Prior to this patch, we were *unable* to pull anything from a
repository with 200 tags.  This is the extent of my investigation thus
far.  Unfortunately, this change is probably just a constant factor
reduction in run-time as total network round-trips still grows linearly
with tag count.
README.md: Shift Index discussion to the external-backend section

We use Index for third-party search engines.  For the 'sqlalchemy'
special case, we use SQLAlchemyIndex.
README.md: Document --preload for the SQLAlchemy search index
avoid using same variable twice in the same function
Merge pull request #659 from misho-kr/master

avoid using a variable twice in the same function
Merge pull request #655 from wking/document-preload

README.md: Document --preload for the SQLAlchemy search index
docker-registry-driver-azure config and docs

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>
Merge pull request #664 from ahmetalpbalkan/master

Azure Storage Driver config and README updates
more docs for azureblob flavor (#664)

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>
Reduce image size by 40MB

Remove upgrade and cleanup after installation to reduce image size.
move azureblob configuration to ADVANCED.md

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>
Merge pull request #666 from stp-ip/patch-1

Reduce image size by 40MB
Merge pull request #665 from ahmetalpbalkan/master

more docs for azureblob flavor
Update Authors
Merge pull request #667 from stp-ip/patch-1

Update Authors (related to #666)
Use -e flag instead of -p flag in advanced example
Fix whitespace
Merge pull request #668 from stp-ip/patch-2

Use -e flag instead of -p flag in advanced example
Fixes issue with s3/boto/gevent hanging.
fixed link to advanced docs in faq