Alerts suddenly start getting inhibited

This evening, the alert node suddenly started inhibiting alerts even there is no .inhibit() property defined. Events from influxdb continued to be received by kapacitor.

Tick Script Template:
```
var measurement string

var where_filter = lambda: TRUE

var groups = [*]

var window = 10s

var deadman_interval = 900s

var topic string

var info = lambda: TRUE

var warn = lambda: FALSE

var crit = lambda: FALSE

var data_raw = stream
|from()
.measurement(measurement)
.where(where_filter)
.groupBy(groups)

data_raw
|alert()
.category('threshold')
.info(info)
.warn(warn)
.crit(crit)
.topic(topic)
[...]
```

Variables:
```
Vars:
Name Type Value
crit lambda "used_percent" > 90.0
measurement string disk
topic string telegraf_disk
warn lambda "used_percent" > 75.0
```

Graph:
```
digraph disk {
graph [throughput="20.01 points/s"];

stream0 [avg_exec_time_ns="0s" errors="0" working_cardinality="0" ];
stream0 -> from1 [processed="111222"];

from1 [avg_exec_time_ns="0s" errors="0" working_cardinality="0" ];
from1 -> alert2 [processed="111222"];

alert2 [alerts_inhibited="18148" alerts_triggered="93074" avg_exec_time_ns="0s" crits_triggered="0" errors="0" infos_triggered="92811" oks_triggered="0" warns_triggered="263" working_cardinality="352" ];

```
Version:
kapacitor-1.6.5-1.x86_64 on Centos 7, Influxdb is 1.8.10

Data is gathered from `telegraf` and sent to influxdb; kapacitor subscribed to the DB. Any pointers?

Start with the alert() node in the provided TICK script and reproduce the graph showing alerts_inhibited without an inhibit property. Inspect the alert node's inhibition metrics and behavior using the supplied template, variables, and Kapacitor 1.6.5 environment; done means identifying the cause and confirming that alerts are no longer inhibited unexpectedly.

Written by the indexing model from the issue text.