Add checkstyle precommit for hidden/shadowed variables

[dakrone]

Checkstyle has the ability to detect when variables are shadowed by local variables: http://checkstyle.sourceforge.net/config_coding.html#HiddenField

We should enable this as it makes reading the code clearer, as well as reducing potential bugs with missing this. prefixes.

[jasontedor]

I assume that we would configure it so that this does not apply to constructor parameters nor to setter parameters?

[dakrone]

Yep, those are options we can definitely enable

[elasticmachine]

Pinging @elastic/es-core-infra

[nik9000]

@dakrone, do you still think this is worth it?

[pugnascotia]

Do we still want to do this? I enabled the HiddenField rule and even after tweaking the settings, it finds a vast number of errors.

We could enable it as a warning, or an IDE-only warning?