Make BridgeDB export internal metrics.

See merge request tpo/anti-censorship/bridgedb!8

BridgeDB already exports usage metrics that help us understand how our
users interact with BridgeDB.  This commit adds internal metrics, which
shed light on BridgeDB's internals.  In particular, we add the following
internal metrics:

* Number of bridges per distributor subring.

* Number of empty responses per distributor.

* Number of IPv4/IPv6 handouts.

* Descriptive statistics on the number of bridge handouts (min, max,
  median, quartile 1, quartile 3, upper whisker, lower whisker).

* Number of unique bridges that were handed out.

This patch fixes tpo/anti-censorship/bridgedb#31422 and also bumps
BridgeDB's metrics format to version 2.

Parse bridge blocking info from SQL database.

See merge request tpo/anti-censorship/bridgedb!7

The plan currently is that wolpertinger will populate our SQL database
with bridge blocking info provided by OONI.  This patch adds code that
parses this data and adds it to our existing bridge objects.

This fixes tpo/anti-censorship/bridgedb#34260.

Remove coverage and pypi link; update Travis CI.

See merge request tpo/anti-censorship/bridgedb!6

Our Travis CI link still pointed to sysrqb's profile.  This commit
updates the link to use phw's profile.  I'm also removing the (outdated)
pypi and coverage links.

This fixes tpo/anti-censorship/bridgedb#40002.

Remove the --reload command line switch.

See merge request tpo/anti-censorship/bridgedb!4

This fixes tpo/anti-censorship/bridgedb#40001.