Performance improvement 1: queryplans

[msm-code]

While "Query Plans" sound like an advanced feature, this is actually an regression from QueryGraphs. Even more - after this PR query graphs will be unused and we could remove them in principle.

This PR reduces the number of reads between 10% and 300%. As seen on this screenshot (ignore time in ms, it's related to disk load and cache contents at the benchmark moment - read count is what matters):

Since read is the most time consuming part of ursadb work, especially on HDD, this may speed up the database significantly.

In this PR I rewrite the matching algorithm to use a minimal covering set of necessary ngrams. For example, to match:

abcde?ghi

(where ? in an unknown/missing character), we will check the following ngrams (assuming all indexes are available):

abcd
 bcde
      ghi

While in the past we would check:

abcd
 bcde
abc
 bcd
  cde
abcd (hash4)
 bcde (hash4)
      ghi

and AND the results later. most of these are unnecessary - for example there's no point checking 3grams abc and bcd since they're implied by 4gram abcd. The same goes for pseudo-4gram abcd provided by the hash4 index - it's implied by text4 one.

The downside is that it's hard to combine this with querygraphs. As mentioned in the PR introducing querygraphs, they're a nice data structure that enables a lot of optimisation - but I never did follow up, so they're quite slow in practice. The only thing we use them for right now are nocase strings. In this case this PR introduces a regression (nocase strings are ignored again). I think this is worth it for a much better performance in a general case.

Full benchmark results here: https://raw.githubusercontent.com/msm-code/ursa-bench/master/results/hdd_all.html (download and open with a browser of your choice).