An AR ConnectionSpec `wait_timeout` is pre-patch used for three
* mysql2 uses it for MySQL's own wait_timeout (how long MySQL
should allow an idle connection before closing it), and
defaults to 2592000 seconds.
* ConnectionPool uses it for "number of seconds to block and
wait for a connection before giving up and raising a timeout error",
default 5 seconds.
* ConnectionPool uses it for the Reaper, for deciding if a 'dead'
connection can be reaped. Default 5 seconds.
Previously, if you want to change these from defaults, you need
to change them all together. This is problematic _especially_
for the mysql2/ConnectionPool conflict, you will generally _not_
want them to be the same, as evidenced by their wildly different
defaults. This has caused real problems for people #6441#2894
But as long as we're changing this, forcing renaming the
ConnectionPool key to be more specific, it made sense
to seperate the two ConnectionPool uses too -- these two
types of ConnectionPool timeouts ought to be able to be
changed independently, you won't neccesarily want them
to be the same, even though the defaults are (currently)