[patch] enable use of refcount policy for ext_data

[Jules Bergmann]

Comparing our vector-add performance (using IPP) against IPP directly 
showed that we had some overhead for small vector sizes (for vector 
sizes less than 1024 elements, our red line falls below IPP's green 
line).  This overhead appears to be from incrementing and decrementing 
reference counts for the blocks being used.  This is being done by 
Ext_data when getting a pointer to the block's data to pass to IPP. 
Ext_data takes a policy template parameter to indicate whether reference 
counting should be done, but it was being ignored and reference counting 
always done.

This patch adds a mechanism to View_block_storage to hold a reference 
using according to a reference-counting policy.

With this patch, our performance (blue line) is closer to IPP for small 
vector sizes.

Patch applied.

				-- Jules
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