BAC-1227: Serve E-Tag along with Last-Modified header

Backport #2583

Conflicts:

	includes/wikia/Wikia.php
Merge pull request #2587 from Wikia/bac-550-served-by-info-in-headers

BAC-550: move "served by" info to header

Backport #2587
Merge pull request #2595 from Wikia/beacon-tracker-send-cb-value

BAC-1157: send CB value with beacon requests

Backport #2595
Merge pull request #2596 from Wikia/release-206-varnish-caching

Varnish caching changes backported