Implement optional text indexing for faster queries

[kshpytsya]

UPD: I see there is a related pull request #123

Consider implementing of text indexing for faster queries, as described here:

https://blog.nelhage.com/2015/02/regular-expression-search-with-suffix-arrays/

Optional because there is an obvious trade-off between storage and querying performance, with winner being dependant on use case. If topics get implemented, this should probably be configurable per topic.
There should also be a way (probably via cli), to index existing logs post factum, as well as a way to delete indexes. This would be useful for performance analysis.

P.S. Forgive me if this has already been considered and rejected -- I could not find anything relevant in Issues or design documents.