Parse new padding-counts lines

As found in today's CollecTor logs:

2017-05-10 06:09:07,711 WARN o.t.c.b.SanitizedBridgesWriter:1212 Unrecognized line 'padding-counts 2017-05-10 01:48:43 (86400 s) bin-size=10000 write-drop=10000 write-pad=10000 write-total=10000 read-drop=10000 read-pad=10000 read-total=70000 enabled-read-pad=0 enabled-read-total=0 enabled-write-pad=0 enabled-write-total=0 max-chanpad-timers=0'. Skipping.

We should provide support for parsing these lines in relay descriptors and in bridge descriptors if we keep those lines there (#22216 (moved)).

Optimistically aiming for 1.7.0.