keeper: fix incorrect parsing of synchronous_standby_names for postgres 9.6.
keeper: fix incorrect parsing of synchronous_standby_names for postgres 9.6.

Fixes #242.

siepkes/stolon,sorintlab/stolon,siepkes/stolon,smkingsoft/stolon,sgotti/stolon,smkingsoft/stolon,sgotti/stolon,sorintlab/stolon