Minimize use of ancillary files

We've traditionally had some ancillary files at ~/.cache/tracker3/files to carry some information across tracker-miner-fs-3 runs:

• no-need-mtime-check.txt to (in the past) avoid mtime checks during crawling in some circumstances. This could lead to great inconsistencies and the theoretical performance benefits were rendered moot with the many performance improvements. So this file was not doing anymore what it meant to do.
• first-index.txt to know whether it should hurry up indexing while on battery or not (for index-on-battery-first-time setting). Nowadays, there is no separation between store and miner, and the database directory is owned (and created) by the miner. This is also a valid check to detect whether tracker-miner-fs-3 is starting from scratch.
• last-crawl.txt to implement crawl-interval in days.

This MR removes the need for the first two, and shuffles the code handling the last one out of the TrackerMinerFiles object, where it's not a great fit.