Ability to run HEAD on _info

[kyleterry]

• storage engine changes that discards values before returning _info
• support for HEAD?

[qpfiffer]

Just to add some detail here: I use this on waifu, but only to get the counts of certain prefices matched. Would be cool if I didn't need to make Oleg do all that work.

[qpfiffer]

So this touches pretty much everything.

• Storage engine modification that will just count the number of prefix matches
  ** Modify ol_prefix_match to return early if there is no data parameter. Just return the number of matches.
• Addition of a verb in the frontend code (HEAD)
• Addition of bindings from the frontend to the modified prefix matching code