perf: http layer - speedup couchdb version lookup
perf: http layer - speedup couchdb version lookup

It turned out that we spend a lot of our time for every request in the
function couch_server:get_version which simply gets the current CouchDB
version.

Instead of fetching a list of all loaded applications an filtering it
we are directly getting the current VSN of CouchDB.

Patched version is 10% faster for a simple /get on a document.

(Almost?) All our of request-handlers are accessing this function so it
should be beneficial for large areas of the http layer.

alanning/couchdb-couch,cloudant/couchdb-couch,ngpestelos/couchdb-couch,apache/couchdb-couch,ngpestelos/couchdb-couch,smithsz/couchdb-couch,alanning/couchdb-couch,smithsz/couchdb-couch,cloudant/couchdb-couch,apache/couchdb-couch,KlausTrainer/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,alanning/couchdb-couch,fkaempfer/couchdb,cloudant/couchdb-couch,smithsz/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,apache/couchdb-couch,ngpestelos/couchdb-couch,KlausTrainer/couchdb-couch,KlausTrainer/couchdb-couch