Now that formatting the buffer-stats string is separate from writing
it to disk, we can also decouple the logic to extract stats from
circuits and finally write some unit tests for the history code.

The new rep_hist_format_buffer_stats() generates a buffer-stats string
that rep_hist_buffer_stats_write() writes to disk.  All the state
changing (e.g., resetting the buffer-stats history and initializing
the next measurement interval) takes place in
rep_hist_buffer_stats_write().  That allows us to finally test the
buffer-stats code better.

We later want to split this function into one function that generates
the buffer-stats string and one that writes it to disk.

So far, if we didn't see a single circuit, we refrained from
generating a cell-stats string and logged a warning.  Nobody will
notice the warning, and people will wonder why there's no cell-stats
string in the extra-info descriptor.  The better behavior is to
generate a cell-stats string with all zeros.

Right now, we append statistics to files in the stats/ directory for
half of the statistics, whereas we overwrite these files for the other
half. In particular, we append buffer, dirreq, and entry stats and
overwrite exit, connection, and bridge stats.

Appending to files was useful when we didn't include stats in extra-info
descriptors, because otherwise we'd have to copy them away to prevent
Tor from overwriting them.

But now that we include statistics in extra-info descriptors, it makes
no sense to keep the old statistics forever. We should change the
behavior to overwriting instead of appending for all statistics.

Implements #2930.

Previously we'd just looked at the connection type, but that's
always CONN_TYPE_AP.  Instead, we should be looking at the type of
the listener that created the connection.

Spotted by rransom; fixes bug 3636.

We'll still need to tweak it so that it looks for includes and
libraries somewhere more sensible than "where we happened to find
them on Erinn's system"; so that tests and tools get built too;
so that it's a bit documented; and so that we actually try running
the output.

Work done with Erinn Clark.

Patch from "blueness".

(that is, to change the default for "UseOptimisticData auto" to
1 once we are more convinced that it works correctly.)

Otherwise, we'll fail, since "9050" looks like a perfectly fine address.

Previously, if tor_addr_to_str() returned NULL, we would reuse the
last value returned by fmt_addr().  (This could happen if we were
erroneously asked to format an AF_UNSPEC address.)  Now instead we
return "???".

The conflicts are with the proposal 171 circuit isolation code, and
they're all trivial: they're just a matter of both branches adding
some unrelated code in the same places.

Conflicts:
	src/or/circuituse.c
	src/or/connection.c

The problem was that we weren't initializing want_length to 0 before
calling parse_socks() the first time, so it looked like we were
risking an infinite loop when in fact we were safe.

Fixes 3615; bugfix on 0.2.3.2-alpha.

Back when I added this logic in 20c0581a, the rule was that whenever
a circuit finished building, we cleared its isolation info. I did that
so that we would still use the circuit even if all the streams that
had previously led us to tentatively set its isolation info had closed.

But there were problems with that approach: We could pretty easily get
into a case where S1 had led us to launch C1 and S2 had led us to
launch C2, but when C1 finished, we cleared its isolation and attached
S2 first.  Since C2 was still marked in a way that made S1
unattachable to it, we'd then launch another circuit needlessly.

So instead, we try the following approach now: when a circuit is done
building, we try to attach streams to it.  If it remains unused after
we try attaching streams, then we clear its isolation info, and try
again to attach streams.

Thanks to Sebastian for helping me figure this out.

One-hop dirconn streams all share a session group, and get the
ISO_SESSIONGRP flag: they may share circuits with each other and
nothing else.

Anonymized dirconn streams get a new internal-use-only ISO_STREAM
flag: they may not share circuits with anything, including each other.