Beats input stops receiving messages even if input is running and accepting connections

## Expected Behavior
Input should not stop receiving messages or at least log an error when this happens.

## Current Behavior
Nodes stop receiving messages and any error is not logged. TCP port used by Beats input is responding when connecting with telnet. There are also many open connections to that TCP port established from clients but no log events are received.
All of ProcessBufferProcessors are in "idle" state.
Thread dump attached.
[graylog_threaddump.txt](https://github.com/Graylog2/graylog2-server/files/6335425/graylog_threaddump.txt)

```
{
"ProcessBufferProcessor #0": "idle",
"ProcessBufferProcessor #1": "idle",
"ProcessBufferProcessor #2": "idle",
"ProcessBufferProcessor #3": "idle",
"ProcessBufferProcessor #4": "idle",
"ProcessBufferProcessor #5": "idle",
"ProcessBufferProcessor #6": "idle",
"ProcessBufferProcessor #7": "idle",
"ProcessBufferProcessor #8": "idle",
"ProcessBufferProcessor #9": "idle",
"ProcessBufferProcessor #10": "idle",
"ProcessBufferProcessor #11": "idle",
"ProcessBufferProcessor #12": "idle",
"ProcessBufferProcessor #13": "idle",
"ProcessBufferProcessor #14": "idle",
"ProcessBufferProcessor #15": "idle",
"ProcessBufferProcessor #16": "idle",
"ProcessBufferProcessor #17": "idle",
"ProcessBufferProcessor #18": "idle",
"ProcessBufferProcessor #19": "idle",
"ProcessBufferProcessor #20": "idle",
"ProcessBufferProcessor #21": "idle",
"ProcessBufferProcessor #22": "idle",
"ProcessBufferProcessor #23": "idle",
"ProcessBufferProcessor #24": "idle",
"ProcessBufferProcessor #25": "idle",
"ProcessBufferProcessor #26": "idle",
"ProcessBufferProcessor #27": "idle",
"ProcessBufferProcessor #28": "idle",
"ProcessBufferProcessor #29": "idle",
"ProcessBufferProcessor #30": "idle",
"ProcessBufferProcessor #31": "idle",
"ProcessBufferProcessor #32": "idle",
"ProcessBufferProcessor #33": "idle",
"ProcessBufferProcessor #34": "idle",
"ProcessBufferProcessor #35": "idle"
}
```

## Possible Solution
Situation is always resolved when restarting Graylog service.

## Your Environment

* Graylog Version: 3.3.11
* Java Version: openjdk 1.8.0.282
* Operating System: RHEL7