influxql: fix WHERE TAG KEY parsing

[dgnorton]

InfluxQL now supports a special TAG KEY identifier in expressions...

SHOW TAG VALUES FROM cpu WHERE TAG KEY = 'host' AND region = 'uswest';

SHOW TAG VALUES FROM cpu WHERE region = 'uswest' AND TAG KEY = 'host';

-- And, for people who are weird...
SHOW TAG VALUES FROM cpu WHERE region = 'uswest' AND 'host' = TAG KEY;

[pauldix]

lgtm, merging now because I need :)