BridgeDB is parsing PTs from `cached-extrainfo*` files cumulatively

This is causing a bridge whose extrainfo descriptor is in both cached-extrainfo and cached-extrainfo.new to have its transport lines added twice to BridgeDB's list of pluggable transports. This is likely what was causing legacy/trac#9264 (moved).

This is easily reproducible: simply generate some fake descriptors, copy the cached-extrainfo.new file to cached-extrainfo (each fake descriptor currently always has exactly two transport lines -- one obfs2 and one obfs3), and start bridgedb:

git clone https://gitweb.torproject.org/bridgedb.git && cd bridgedb && make install
pip install leekspin
generate-OR-descriptors -n 50
cp cached-extrainfo.new cached-extrainfo
bridgedb

If you do tail -f bridgedb.log, you'll see the following lines confirming the bug:

Mar 14 04:14:14 [DEBUG] 836f58807dc44a248c684da0be18b918badef421 supports 4 transports
Mar 14 04:14:14 [DEBUG] e79da1d32ad16e000149449ea191a0e4baa32ca0 supports 4 transports
Mar 14 04:14:14 [DEBUG] 8de1a8fb8c0e2d1885fd05e39541a50a507685ff supports 4 transports
Mar 14 04:14:14 [DEBUG] d0ee79b3bad89ebe6cb1e833e8ba8003ad315e06 supports 4 transports
Mar 14 04:14:14 [DEBUG] Saving state to:        '/home/isis/code/torproject/bridgedb/run-manual/bridgedb.state'