Add Storage Policy Support to Account HEAD

This change updates the account HEAD handler to report out per
policy object and byte usage for the account.  Cumulative values
are still reported and policy names are used in the report
out (unless request is sent to an account server directly in
which case policy indexes are used for easier accounting).

Below is an example of the relevant HEAD response for a cluster
with 3 policies and just a few small objects:

   X-Account-Container-Count: 3
   X-Account-Object-Count: 3
   X-Account-Bytes-Used: 21
   X-Storage-Policy-Bronze-Object-Count: 1
   X-Storage-Policy-Bronze-Bytes-Used: 7
   X-Storage-Policy-Silver-Object-Count: 1
   X-Storage-Policy-Silver-Bytes-Used: 7
   X-Storage-Policy-Gold-Object-Count: 1
   X-Storage-Policy-Gold-Bytes-Used: 7

Change-Id: I5ec251f9a8014dd89764340de927d09466c72221
Implements: blueprint storage-policies