Replication Key Signpost Behavior not consistent with expected behavior
## Summary
[//]: #
My understanding was that the `get_replication_key_signpost` method is used to make sure that the replication key value that is stored is not above a certain value right? so for example, I overwrote the method to return 2021-08-04T19:08:35+00:00 (as a datetime object), but I see that the replication key value that is stored in my state is 2021-08-04T19:17:29Z which is greater than the replication key signpost value. Am I misunderstanding what the behavior of that method should be?
## Steps to reproduce
[//]: #
- Ingest a stream with any timestamp field that will be used for replication (can generate a stream with fake data)
- In your stream class, override the `get_replication_key_signpost` method to return a datetime that is earlier than the highest date in your fake data
- Make sure the stream is unsorted
- Run ELT with a job_id, so state gets saved
- Check the value of the replication_key in state, it will not be the `get_replication_key_signpost` value, but the highest value in the stream fake data
## What is the current bug behavior?
[//]: # The timestamp of a stream stored in the state can be higher than what the `get_replication_key_signpost` method returns
## What is the expected correct behavior?
[//]: # The timestamp of a stream stored in state should never be higher than what `get_replication_key_signpost` returns
## Relevant logs and/or screenshots
[//]: # An example, is I ingested a stream that outputted the following data:
```json
{"id": "1112931637383", "updatedAt": "2021-08-04T16:38:04Z" }
{"id": "1112931637323", "updatedAt": "2021-08-04T03:58:39Z"}
{"id": "111293163735", "updatedAt": "2021-08-04T11:25:30Z"}
{"id": "1112931637", "updatedAt": "2021-08-04T16:37:54Z"}
{"id": "1112931637343", "updatedAt": "2021-08-04T10:42:30Z"}
{"id": "1112931637393", "updatedAt": "2021-08-04T09:44:58Z"}
{"id": "111293163738211", "updatedAt": "2021-08-04T08:36:52Z"}
{"id": "11129316373812", "updatedAt": "2021-08-04T07:55:35Z"}
{"id": "11129316373817", "updatedAt": "2021-08-04T07:24:55Z"}
{"id": "11129316373892", "updatedAt": "2021-08-04T09:53:29Z"}
```
State File
```json
{
"bookmarks": {
"fake_stream": {
"replication_key": "updatedAt",
"replication_key_value": "2021-08-04T16:38:04Z"
}
}
}
```
However the value of replication_key_signpost was `2021-08-02T00:00:00+00:00`
## Possible fixes
[//]: # Looks like this [method](https://gitlab.com/meltano/sdk/-/blob/main/singer_sdk/streams/core.py#L522) gets the signpost and passes it to this [method](https://gitlab.com/meltano/sdk/-/blob/main/singer_sdk/helpers/_state.py#L188), but it is never use
issue