Until now, we had no insight into how BridgeDB is being used.  We don't
know the relative popularity of our distribution method; we don't know
how many users BridgeDB sees; we don't know how many requests succeed or
fail; and we don't know the relative popularity of transports that users
request.

This patch attempts to answer these questions by making BridgeDB export
usage metrics.  At the end of each 24-hour measurement interval,
BridgeDB will append usage metrics to the file METRICS_FILE, which is
configured in bridgedb.conf.

Our metrics keep track of the number of (un)successful requests per
transport type per country code (or email provider) per distribution
method.  This way, we get to learn that, say, over the last 24 hours
there were 31-40 users in Iran who successfully requested an obfs4
bridge over Moat.  The corresponding metrics line would look as follows:

  bridgedb-metric-count moat.obfs4.ir.success.none 40

To make the metrics preserve user privacy, we don't collect
user-identifying information and we introduce noise by rounding up
metrics to our bin size which defaults to 10.

This patch also extends the looping calls that BridgeDB spawns.  When
BridgeDB first starts, it loads proxies from the files PROXY_LIST_FILES.
It augments this list of proxies with Tor exit relays that we download
every three hours.