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Export Prometheus-based metrics

Closes #4

See merge request tpo/anti-censorship/bridgestrap!3

The way Prometheus works is that a central instance scrapes metrics from
various endpoints, bridgestrap being one of them.  We expose a new HTTP
handler for /metrics and let our Prometheus library do the rest.

This patch fixes tpo/anti-censorship/bridgestrap#4

So far, bridgestrap would serialise requests by relying on a mutex's
locking mechanism.  That's dirty.  This patch implements a channel-based
dispatching mechanism that guarantees order (mutexes don't).  In
addition to that, we can now log the number of outstanding requests,
which is good to know.

Having tor log at 'info' level makes its log files grow significantly
faster.

So far, our cache timeout was hard-coded to be one week.  Now that
bridgestrap is more efficient, we can afford to test resources more
often, so this patch changes the default timeout to 24 hours.  The patch
also makes this timeout configurable through the new -cache-timeout
switch.

While at it, the patch also refactored our cache mechanism a bit and
renamed -timeout to -test-timeout.

This patch fixes #8

This patch adds the -log command line switch, which instructs
bridgestrap to log to the given file.

This patch bumps our code coverage to 48.6%.

For reasons outlined in #2, we want to be
able to use a custom tor for now.  This patch introduces a new command
line switch, -tor, which makes this possible.

Issue/6

Closes #6

See merge request !2

This patch fixes tpo/anti-censorship/bridgestrap#6

...and start with version 0.0.1.

If our event channel is closed, our last read results in nil.  This
patch makes bridgestrap check for this condition before accessing the
result's struct members.

We can safely ignore http.ErrServerClosed but we really ought to check
for other errors.

Issue/3

Closes #3

See merge request !1

So far, bridgestrap would bootstrap a new tor instance for each request.
That's both expensive and slow.  Roger suggested to use a single,
long-lived tor instance instead, and use the SETCONF controller command
to test a batch of bridges at once.  This patch makes that happen.

This fixes tpo/anti-censorship/bridgestrap#3.

Simply run:

  ./scripts/test-bridge-lines /path/to/bridge/file

The provided file must contain bridge lines, one per line.

Thanks to Cecylia for questioning whether the cache lookup mechanism is
fast enough:
ecc6e048 (comment 2709958)

We currently have ~1,700 bridges, so a buffer size of 1,000 isn't going
to cut it if we ask bridgestrap to test our entire pool of bridges at
once.

This command line option instructs bridgestrap to only pretty-print its
cache and then exit.

Unlike the Web API, we don't reject connections.  Instead, we try hard
to serve them all, even though some may be delayed.

That allows us to cache bridge lines of connections that were still open
while we received the signal to shut down.