db: force _id index for capped log collection
db: force _id index for capped log collection

After removing this index we started having problems with replication.
Secondary MongoDB servers crashed after lots of failed replications.

tsuru/tsuru,ArxdSilva/tsuru,kgrvamsi/tsuru,kgrvamsi/tsuru,tuxmonteiro/tsuru,cezarsa/tsuru,cezarsa/tsuru,gwmoura/tsuru,hcxiong/tsuru,4eek/tsuru,kgrvamsi/tsuru,guilhermebr/tsuru,hcxiong/tsuru,cgvarela/tsuru,firebitsbr/tsuru,tuxmonteiro/tsuru,mderaldo/tsuru,tuxmonteiro/tsuru,guilhermebr/tsuru,Endika/tsuru,gciotta/tsuru,cezarsa/tsuru,ggarnier/tsuru,tsuru/tsuru,Endika/tsuru,4eek/tsuru,ArxdSilva/tsuru,cezarsa/tsuru,guilhermebr/tsuru,ArxdSilva/tsuru,4eek/tsuru,ggarnier/tsuru,hcxiong/tsuru,gciotta/tsuru,Endika/tsuru,gciotta/tsuru,cgvarela/tsuru,tsuru/tsuru,ggarnier/tsuru,gwmoura/tsuru,mderaldo/tsuru,firebitsbr/tsuru,mderaldo/tsuru,tsuru/tsuru,firebitsbr/tsuru,cgvarela/tsuru,ArxdSilva/tsuru,gwmoura/tsuru,ggarnier/tsuru,guilhermebr/tsuru