query influxdb directly for series, remove maintain_cache.py

now influxdb supports "list series /regex/",
so we don't need maintain_cache.py anymore,
we don't need to load all series in the cache,
and we don't need to do regex filtering in python anymore.

advantages: faster, simpler !

here are some measured statsd timings ( mean ~ upper) in ms

before:
if series not cached:
    cache_get_nodes # 100~200
    find_series # 68 ~ 100
everything included:
yield_nodes # 130 ~ 2000

after:
if series not cached:
    (cache_get_nodes and find_series not more used)
    get_series based on influxdb list series: # 150 ~ 300
everything included:
yield_nodes # 45 - 350

so worst case performance has improved considerably,
typical performance has improved a bit.

performance graph: http://i.imgur.com/gDfqD1R.png

NOTE: this only works with cheat_times enabled, which you should have
anyway, there's no point in disabling it, see next commit
remove cheat_times

honestly we can just pretend we have the data from beginning of time
until now, always.  missing data just shows up as None.
I haven't seen that causing any issues, i've always ran with cheat_times
enabled.

once we have something in place to get insights into retention policies,
then we can use that, or maybe if influx has a fast way to gave is this
info.

this will also improve performance for those who had cheat_times
disabled