Implement mongodb query syntax for task filters

[msm-cert]

This adopts code from https://github.com/kapouille/mongoquery to implement a mongodb-like syntax for task filtering in karton.

As mentioned in the code, there are some pitfalls since the old negative matching works in three-state logic and can't be translated to the new query syntax literally. For a short example:

[
  {"platform": "!win32"},
  {"platform": "!linux"}
]

will match all non-linux non-windows samples, as was probably intended, but:

[
  {"platform": {"$not": "win32"}},
  {"platform": {"$not": "linux"}}
]

is wrong - this means literally task.headers["platform"] != "win32" or task.headers["platform"] != "linux" and hence will match every task.

To get equivalent behaviour with mongo syntax, you should use:

[{
  "platform": {
    "$not": { "$or": ["win32", "linux"] }
  }
}]

See #246 for original reasoning behind the old-style behaviour.

Background compatibility considerations

1. "Old style" filters should keep working (any regression here is a bug). We don't plan to remove or obsolete that syntax in the forseeable future. No migration is necessary.
2. It's not recommended to mix mongodb-style filters and negative matches, to avoid unexpected behaviour. We may actually want to explicitly check it and forbid it?
3. "Nesting" old style wildcards and mongodb syntax doesn't work. "platform": {"$or": ["win*", "linux*"]} will only work for a win* literal, for example.

[psrok1]

Repeated negative_filter definition

Suggested change

	regular_filter, negative_filter = [], []
	negative_filter = []
	regular_filter, negative_filter = [], []

[psrok1]

closes #245

[psrok1]

Looks good overall: it would be nice to have an entry in documentation about new syntax and its limitations.