update mysql library: 2.6.1 -> 2.15.0
update mysql library: 2.6.1 -> 2.15.0

version 2.6.1 of mysql library is from 2015, and has a Remote Memory Exposure vulnerability (see https://github.com/ether/etherpad-lite/issues/3397).

This change updates the library at its latest version as of today (2.15.0).
Tested against mysql 5.7 with no evident regressions.
Pita/ueberDB