Add support for if-modified-since header#1

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Nyoroon wants to merge 3 commits intoprerender:masterfrom

Commits on Apr 4, 2019

Add handling of if-modified-since header
Ivan Kalinin
committed

Commits on Apr 11, 2019

Support for custom endpoint
Ivan Kalinin
committed

Commits on Apr 15, 2019

Compress cache
Ivan Kalinin
committed