Counter cache column

[kbrock]

code was pulled out of #600

• Drop unnecessary _counter_cache_column, and remove all the to_s that were used.

May merge with #618 - but at this time, refactoring the code and fixing the counters that get askew seem like different problems

[kbrock]

@kshnurov A few questions:

• was there a particular use case you were protecting by adding the parent_id checks?
• It looks like those checks are already made in the callback definitions.

I did notice that ancestry_callbacks_disabled? was not checked in the increase_parent_counter_cache. Not sure if these are the root of #618. Will probably move from this PR and put into that one since it is more of a potential bug fix rather than a refactor.

[kbrock]

update:

• dropping parent_id checks from callbacks. leaving that code in fix increase_parent_counter_cache callback #618 so we can apply if it is necessary