Implement query graph pruning

[msm-code]

Ignore query graph paths that are "too branched".

Intuitively, When we have something like {AA BB CC DD ?? ?? ?? ?? ?? ?? EE FF GG HH} we should just split this into {AA BB CC DD} & {EE FF GG HH}.

Implementation-wise, compute branching level (using ngram density estimation) for every node, and collapse nodes where it's too big.

This is a blocking issue for releasing query graphs, because without it some queries will not be stable enough.

[msm-code]

Temporarily solved with #73