Optimize search distance

[ezorita]

Reduce max tau during trie search if a sequence of tau < tau_max is found.

For instance, for a trie search of 3 mismatches, a sequence of distance 1 is found. Since we're only interested in the closest neighbor, set tau_max = 1 for the rest of this search.

[ezorita]

To maximize the probability of finding such close sequence as fast as possible, start the search from the right of the trie and extend from right to left, instead of doing left-right search.