Make visitors threadsafe by removing @last_column

The last_column feature of the ToSql visitor and its descendants is what
enabled quoting based on the column last visited -- in other words, if
you have a standard condition like an equality with a string attribute
on the left side and an integer on the right side, then when ARel visits
the node, it'll first visit the left side attribute, setting the
column of the string attribute as the last column, and resulting in the
right side of the condition getting the appropriate quoting.

The downside is that this means that visitors can't be shared between
threads, because of the state mutation. It also makes for some really
weird behavior in the event that the visitor visits a node that happens
to contain an attribute you weren't expecting to be there, since it'll
potentially quote something based on that attribute. So, it prevents
reversing an equality condition. column = value will work, but not value
= column, since the last column wouldn't be the column you're hoping
for.

This is a first pass at fixing this by changing the signature of the
visit methods to accept the currently-relevant attribute, if any.