Listen for Mongo change streams updates and rebuild query cache

Closes #13 and #14

See merge request !6

Allow arbitrary policy package names

Closes #19

See merge request !5

HTTP endpoints for policy lock and unlock

Closes #10 and #11

See merge request !3

HTTP endpoint for policy evaluation (execution)

Closes #12

See merge request !2

This is an experiment to produce a generic evaluation
function for different polices, which is able to return
arbitrary JSON result after policy execution. For the
moment a lot of conventions have to be followed in order
for this to work, which is not so good. More experiments
will follow to reduce the conventions to minimum (it would
be best to remove all of them).

Goa skeleton with health check endpoints

Closes #7

See merge request !1