Attempt some performance tweaks

Try to tweak the MySQL train_station_list() stored procedure and the
associated database retrieval code.

Phase out DateTime::Format::Strptime because it's slow and can be easily
replaced with a rather dumb yet faster regular expression.

--HG--
branch : perf
Save last visited station in cookie

Save station in cookie whenever user changes it.  That way, the last-visited
station is recalled at the next visit unless a "s" parameter already exists in
the URL.
Add Bréval station to database
Clean up code and add tests for /autocomp
Merge branch 'cookie' into 'default'
Merge branch 'perf' into 'default'
Remove debug statements; rename RER::Trains::format to TO_JSON

This method was necessary anyway; otherwise the /json route simply returns
"null" instead of our JSON-encoded text.