store: store a single charm for all its urls
store: store a single charm for all its urls

It's clear now that storing multiple charms just because
they have a different URL is unnecessary.

Also add index for events collection.

urosj/charmstore-1,mhilton/charmstore,mhilton/juju-charmstore,jrwren/charmstore,wallyworld/charmstore,wallyworld/charmstore,wallyworld/charmstore,fabricematrat/charmstore,makyo/charmstore,mhilton/juju-charmstore,ericsnowcurrently/juju-charmstore,jrwren/charmstore,mhilton/charmstore,urosj/charmstore-1,ericsnowcurrently/juju-charmstore,makyo/charmstore,urosj/charmstore-1,frankban/charmstore,fabricematrat/charmstore,fabricematrat/charmstore,jrwren/charmstore,mhilton/juju-charmstore,juju/charmstore,juju/charmstore,juju/charmstore,mhilton/charmstore,frankban/charmstore,makyo/charmstore,ericsnowcurrently/juju-charmstore,frankban/charmstore