mongo: don't put fake oplog in "local" DB
mongo: don't put fake oplog in "local" DB

For some reason, if the fake oplog collection used during testing is
in the local DB there are often 1s pauses when reading from the tail
cursor. Putting it in another DB works around the issue.

bz2/juju,davecheney/juju,wwitzel3/juju,macgreagoir/juju,fwereade/juju,bogdanteleaga/juju,mjs/juju,cloud-green/juju,perrito666/juju,tasdomas/juju,gabriel-samfira/juju,voidspace/juju,bz2/juju,waigani/juju,mjs/juju,kat-co/juju,howbazaar/juju,davecheney/juju,tych0/juju,thomnico/juju,bz2/juju,reedobrien/juju,mjs/juju,mjs/juju,kat-co/juju,cloud-green/juju,bz2/juju,tych0/juju,ericsnowcurrently/juju,mhilton/juju-juju,bac/juju,perrito666/juju,waigani/juju,axw/juju,wwitzel3/juju,bac/juju,makyo/juju,makyo/juju,marcmolla/juju,mhilton/juju-juju,marcmolla/juju,dooferlad/juju,dimitern/juju,kat-co/juju,cloud-green/juju,waigani/juju,AdamIsrael/juju,howbazaar/juju,frankban/juju,dooferlad/juju,bz2/juju,howbazaar/juju,mhilton/juju,mhilton/juju,frankban/juju,gabriel-samfira/juju,frankban/juju-tmp,perrito666/juju,thomnico/juju,marcmolla/juju,bac/juju,mjs/juju,tsakas/juju,axw/juju,tych0/juju,frankban/juju-tmp,mhilton/juju-juju,bac/juju,mikemccracken/juju,AdamIsrael/juju,fwereade/juju,dimitern/juju,voidspace/juju,dimitern/juju,howbazaar/juju,mhilton/juju,ericsnowcurrently/juju,frankban/juju,thomnico/juju,gabriel-samfira/juju,Altoros/juju-cloudsigma,tsakas/juju,tasdomas/juju,macgreagoir/juju,mikemccracken/juju,tasdomas/juju,mikemccracken/juju,bac/juju,Altoros/juju-cloudsigma,AdamIsrael/juju,gabriel-samfira/juju,bogdanteleaga/juju,mhilton/juju-juju,mjs/juju,marcmolla/juju,voidspace/juju,reedobrien/juju,waigani/juju,alesstimec/juju,dooferlad/juju,mikemccracken/juju,dimitern/juju,reedobrien/juju,perrito666/juju,axw/juju,ericsnowcurrently/juju,tsakas/juju,dooferlad/juju,thomnico/juju,wwitzel3/juju,kat-co/juju,Altoros/juju-cloudsigma,reedobrien/juju,tsakas/juju,howbazaar/juju,axw/juju,AdamIsrael/juju,macgreagoir/juju,reedobrien/juju,davecheney/juju,Altoros/juju-cloudsigma,anastasiamac/juju,anastasiamac/juju,mjs/juju,ericsnowcurrently/juju,fwereade/juju,tasdomas/juju,frankban/juju-tmp,ericsnowcurrently/juju,anastasiamac/juju,alesstimec/juju,makyo/juju,voidspace/juju,anastasiamac/juju,alesstimec/juju,axw/juju,macgreagoir/juju,frankban/juju-tmp,wwitzel3/juju,makyo/juju,anastasiamac/juju,frankban/juju,alesstimec/juju,alesstimec/juju,bogdanteleaga/juju,perrito666/juju,frankban/juju,tych0/juju,davecheney/juju,cloud-green/juju,bogdanteleaga/juju,gabriel-samfira/juju,mhilton/juju,marcmolla/juju,mikemccracken/juju,macgreagoir/juju,dooferlad/juju,fwereade/juju,dimitern/juju,kat-co/juju