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).
db: raise the level of the cache

For better reuse :-)
db: removed Storage.Close method

This method should not be used anymore. We should collect dead
connections and close it.
db: added one level of indirection to mgo.Session

This will allow us to collect old connections. We will mark collections
not used recently, and them cleanup everything :-)
db: added a connection collector

This collector retires unused connections, so they don't feel tired :-)