mgr/influx: Only fetch the current time once when gathering data

mgr/influx: Only fetch the current time once when gathering data

By fetching the current time once and storing it into a variable
we save a lot of system calls. On large clusters this can be a lot
of system calls.

In addition we also make sure that all points gathered in the same
loop/run have exactly the same timestamp.

Otherwise there will be a difference in time between the items in
InfluxDB which then causes problems when creating graphs with for
example Grafana.

Signed-off-by: Wido den Hollander <wido@42on.com>