Equalize data store backends

[glensc]

We have been merging changes that gradually added support for PDO databases. However recently the changes have been shifting towards PDO: #305 supports only PDO.

We should track the differences somehow, so both support can be improved.

Please leave a comment on what you know is missing from one type of data store and I'll update this table.

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Feature	MongoDB	PDO
Prometheus exporter	✗	✓
Searcher::latest()	✓	✓
Searcher::query()	✓	✗ #384
Searcher::get()	✓	✓
Searcher::getForUrl()	✓	✓#436
Searcher::getPercentileForUrl()	✓	✓#436
Searcher::getAvgsForUrl()	✓	✗ #384
Searcher::getAll(sort)	✓	✓#436
Searcher::getAll(direction)	✓	✓#436
Searcher::delete()	✓	✓
Searcher::truncate()	✓	✓
Searcher::saveWatch()	✓	✓#435
Searcher::getAllWatches()	✓	✓#435
Searcher::truncateWatches()	✓	✓#435
Searcher::stats()	✗ (#305)	✓

[glensc]

Probably here's up to date checklist what could be missing:

• Backend decoupling #244

[glensc]

even asked in issue body, so far nobody has left a comment to update the compatibility matrix, so I'm going to close this, the table moved to readme #461