Backporting performance improvements (release-243)

https://wikia-inc.atlassian.net/browse/PLATFORM-431

@Wikia/platform-team 

Performance improvements:

* WatchedItem: avoid select/writes for anon user in WatchedItem #4770
* ImageServing: prevent queries by page_id = 0 #4774

To be pushed live on Monday, September 22nd. We'd like to monitor this performance improvement without any other changes going live at the same time.

```
site> select key, count, time_p50, time_p95, total_time from Database_Queries_60m  where key =~ /WatchedItem::isWatched/  limit 5
┌──────────────────┬─────────────────┬───────┬───────────────────────────────┬─────────────────────┬────────────────────┬────────────────────┐
│       time       │ sequence_number │ count │ key                           │ time_p50            │ time_p95           │ total_time         │
├──────────────────┼─────────────────┼───────┼───────────────────────────────┼─────────────────────┼────────────────────┼────────────────────┤
│ 19/9/14 14:00:00 │ 518             │ 17597 │ select-WatchedItem::isWatched │ 0.3193241578561284  │ 2.264903025862611  │ 11610.924482345577 │
│ 19/9/14 13:00:00 │ 421             │ 19438 │ select-WatchedItem::isWatched │ 0.31848663979388303 │ 2.2772734288386673 │ 12766.570568084719 │
│ 19/9/14 12:00:00 │ 424             │ 17341 │ select-WatchedItem::isWatched │ 0.3128140334374397  │ 1.8310040881390166 │ 9867.2354221344    │
│ 19/9/14 11:00:00 │ 443             │ 17130 │ select-WatchedItem::isWatched │ 0.3194382204811954  │ 2.158705017270417  │ 11358.243703842165 │
│ 19/9/14 10:00:00 │ 517             │ 16717 │ select-WatchedItem::isWatched │ 0.32010003712102525 │ 2.2314249620138775 │ 11907.268047332767 │
└──────────────────┴─────────────────┴───────┴───────────────────────────────┴─────────────────────┴────────────────────┴────────────────────┘

site> select key, count, time_p50, time_p95, total_time from Database_Queries_60m  where key =~ /ImageServing/  limit 10
┌──────────────────┬─────────────────┬───────┬──────────────────────────────────────────────────────────────────┬─────────────────────┬─────────────────────┬────────────────────┐
│       time       │ sequence_number │ count │ key                                                              │ time_p50            │ time_p95            │ total_time         │
├──────────────────┼─────────────────┼───────┼──────────────────────────────────────────────────────────────────┼─────────────────────┼─────────────────────┼────────────────────┤
│ 19/9/14 14:00:00 │ 314             │ 4652  │ select-ImageServingDriverMainNS::getImagesPopularity::redirects  │ 0.6903247063204374  │ 2.997409853135574   │ 5174.968004226685  │
│ 19/9/14 14:00:00 │ 313             │ 4396  │ select-ImageServingDriverMainNS::getImagesPopularity::imagelinks │ 0.9256860484247633  │ 5.951139793632933   │ 8451.340436935423  │
│ 19/9/14 14:00:00 │ 312             │ 9324  │ select-ImageServingDriverMainNS::getArticleProbs                 │ 0.3456899614045116  │ 2.133199662873243   │ 6308.228254318236  │
│ 19/9/14 14:00:00 │ 311             │ 5311  │ select-ImageServingDriverMainNS::filterImages                    │ 0.40786085706768604 │ 2.4001330317872958  │ 4067.7585601806645 │
│ 19/9/14 14:00:00 │ 310             │ 1359  │ select-ImageServingDriverCategoryNS::getCategoryArticleList      │ 0.7376938767075923  │ 3.433263340679749   │ 1850.622177124022  │
│ 19/9/14 14:00:00 │ 309             │ 2847  │ select-ImageServing::getImages                                   │ 0.3365126925725313  │ 1.837714427026798   │ 1780.3039550781243 │
│ 19/9/14 14:00:00 │ 159             │ 20    │ replace-ImageServingHelper::buildIndex                           │ 0.8255441983540872  │ 0.8823891480763764  │ 17.541646957397482 │
│ 19/9/14 14:00:00 │ 30              │ 15    │ delete-ImageServingHelper::buildIndex                            │ 0.3414750099182142  │ 0.36917839731489205 │ 5.4256916046143    │
│ 19/9/14 13:00:00 │ 236             │ 4920  │ select-ImageServingDriverMainNS::getImagesPopularity::redirects  │ 0.7025080224487689  │ 2.983886040061528   │ 5501.692533493043  │
│ 19/9/14 13:00:00 │ 235             │ 5119  │ select-ImageServingDriverMainNS::getImagesPopularity::imagelinks │ 0.8946254940852065  │ 6.836544297224172   │ 9962.917804718028  │
└──────────────────┴─────────────────┴───────┴──────────────────────────────────────────────────────────────────┴─────────────────────┴─────────────────────┴────────────────────┘
Query took  433 ms
```