Postgres 11 compatibility

[NikolayS]

Postgres 11 is to be out tomorrow (Thu, Oct 18), it's time to add compatibility with it. So we'll need to do two steps:

• docker image for 11
• allow --pg-version 11

We still keep 10 as default, but will probably switch the default to 11 very soon to encourage benchmarks, tests, etc.

FYI @dmius @ane4ka @myyorick @michelp @Nastradamus

[NikolayS]

First pgbench results: https://twitter.com/postgresmen/status/1053484604642942976