Account quotas

Add a new middleware implementing account quotas.

This middleware blocks write requests (PUT, POST) if a given quota (in bytes)
is exceeded while DELETE requests are still allowed.

Quotas are stored in the x-account-meta-quota-bytes metadata entry.
Write requests to this metadata setting are only allowed for resellers.

Change-Id: I57fd7c6209f34cc79d4bab72d500d43ba2a62083
Merge "Account quotas"