Negation operators don't work as expected for meta queries

The negation operators (`!=` and `!~`) don't work as expected for some meta queries. The following meta queries are not affected:

```
SHOW SERIES
SHOW TAG VALUES
SHOW FIELD KEYS
```

Here are examples of affected meta queries:

#### Example 1 - SHOW MEASUREMENTS

This example fails on `tip` and also on `1.4.2` and `1.3.6`.
```
> create database db
> use db
Using database db
>
> insert cpu,region=west value=1
> insert cpu,region=east value=1
>
> show measurements
name: measurements
name
----
cpu
>
> show measurements where region = 'west'
name: measurements
name
----
cpu
>
> show measurements where region = 'east'
name: measurements
name
----
cpu
>
> show measurements where region != 'east'
>
> show measurements where region != 'west'
>
```

In both of these cases we should expect the `cpu` measurement to be returned. The issue here is that because there exist a series in the measurement that doesn't satisfy `!= 'east'`, the entire measurement is excluded from the result set. To clarify this a little further:

```
> create database db
> use db
Using database db
>
> insert cpu,region=east value=1
> insert cpu,region=west value=1
> insert mem,region=east value=1
>
> show measurements where region = 'east'
name: measurements
name
----
cpu
mem
>
> show measurements where region != 'west'
name: measurements
name
----
mem
>
>
```
The `!= 'west'` returns `mem` because `mem` does not have a series where `region = west`.

#### Example 2 - SHOW TAG KEYS

```
> drop database db
> create database db
>
> insert cpu,region=west value=1
> insert cpu,region=east value=1
>
> show tag keys
name: cpu
tagKey
------
region
>
> show tag keys where region != 'east'
> show tag keys where region !~ 'west'
ERR: error parsing query: found west, expected regex at line 1, char 30
> show tag keys where region !~ /west/
```

The same problem as `SHOW MEASUREMENTS`. The negation options will exclude any tag key if there is an example series where the predicate is not true.

The issue is to do with the way the [tags are matched](https://github.com/influxdata/influxdb/blob/master/tsdb/index.go#L1310-L1434) when looking for measurements to return.

(For those with access, there was an internal conversation here: https://influxdb.slack.com/archives/C03ARPJ6W/p1510833917000363 back in November.)

Start in tsdb/index.go, particularly the tag-matching logic around lines 1310-1434, and reproduce the SHOW MEASUREMENTS and SHOW TAG KEYS examples from the issue. Trace how != and !~ predicates are applied across series; done means affected meta queries return measurements and tag keys according to the demonstrated negation semantics.

Written by the indexing model from the issue text.