Fix metadata overall limits bug

Currently metadata limits are checked on a per request basis. If
multiple requests are sent within the per request limits, it is
possible to exceed the overall limits.  This patch adds an overall
metadata check to ensure that multiple requests to add metadata to
an account/container will check overall limits before adding
the additional metadata.

This is a backport to the stable/icehouse branch for commit SHA
5b2c27a5874c2b5b0a333e4955b03544f6a8119f.

Closes-Bug: 1365350

Conflicts:
	swift/common/db.py
	swift/container/server.py

Change-Id: Id9fca209c9c1216f1949de7108bbe332808f1045