Review map token expiration renewal code

[strk]

There might be a possible race condition between an expiring map token key and a renewal happening close to expiration. If the code reading the token from redis happens to run before the code renewing the expiration, a "map token not found" error would be triggered at map creation.

Possible reason for the race condition:
https://github.com/CartoDB/Windshaft/blob/0.21.0/lib/windshaft/mapstore.js#L133

See https://groups.google.com/forum/#!topic/cartodb/9TfRcqpuPH4

\cc @rochoa, @javisantana

[strk]

A possible way to test this could be to set serverOptions.grainstore.default_layergroup_ttl to a very low number and create the same map with a frequency close to the expiration time (not easy!).