db: unexport mutex in Storage struct

And some comments preparing to #269.
db: refactor package

The package now uses a connection pool. Now, a user of this package
should always call Open, and let the package manage the connections.

Related to #269.
db: added Conn function

This function reads config file and connects to the database as
specified by the keys database:url and database:name.

Related to #269.
db: added some missing tests
db: added TODO regarding Close deletion

This method should not exist in a pool of connections :-)

Related to #269.
auth: port to new db package model

In this model packages need to store the connection :-)

Related to #269.
service: use the new db package

Related to #269.
app: use new db package

Related to #269.
db: use a rwmutex to protect access to the pool

Related to #269.
provision/juju: use new db package

Related to #269.
all: update license header for files modified this year
collector: adapt code to the new db package

Related to #269.
api: adapt code to the new db package

Related to #269.
api: remove old and dead simple TODO