Add 5.19.0, retire 5.17.x, bump 5.1[246].x to latest releases

There are newer versions in each of 5.16.x, 5.14.x and 5.12.x.

The 5.17.x line is now irrelevant, now that 5.18.0 is released -- new bleeding edge development is now available in 5.19.x.