bug fix in optionalHttpHeaders; deprecated ioutil removed

add HTTP headers: X-Frame-Options, Referrer-Policy

rethinkdb support for hard-deleting p2p messages

Merge branch 'devel'

fix PC language

fixing comments

instructions on creating a private installation

postgress: reduce the number of open connectins to fit under default …

…limit

adding ukrainian translations

Merge pull request #878 from tinode/session-sub-race-dev

Fix a race condition around Session.subs.