Change alert levels to be independent

[nathanielc]

Fixes #298

@beckettsean @rkuchan In reference to this blog post: https://influxdata.com/blog/tldr-influxdb-tech-tips-may-19-2016/ about Alert levels begin subsets. I think it should change, and this PR makes the change so that each alert level is independent of other levels.

Some of my reasons:

• It's initially confusing, since its natural to think in the context of only a single alert level when writing the expression.
• It's become really painful when your data does not follow a continuous domain (aka the data is not sort-able like a set of strings as labels).

Example of non continuous data:

// Data has status flag with values OK, WARN, or CRITICAL
// You have to write the expressions as
|alert()
    .warn(lambda: "status" == 'WARN' OR "status" == 'CRITICAL')
    .crit(lambda: "status" == 'CRITICAL')

This change allows the expressions to be simply

// Data has status flag with values OK, WARN, or CRITICAL
|alert()
    .warn(lambda: "status" == 'WARN')
    .crit(lambda: "status" == 'CRITICAL')

This represent a small breaking change so I want to get it in before the 1.0 official release.
The majority of the uses cases will be unaffected, only uses cases that leveraged the statefulness of expressions and relied on their execution order will break.

Wanted to make you both aware of this change since its been point of discussion before.

• Rebased/mergable
• Tests pass
• CHANGELOG.md updated