db: also reconnect if ping fails
db: also reconnect if ping fails

If the connection is closed, ping panics. If the server dies, ping will
return error.

I'm not able to test this (I could kill the MongoDB process from the
test, but that would break all other tests).

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