Improve Lock Session LockDelay handling inc. lockDelayThreshold

We do some conversion to convert a lock delay of anything less than `1000ns` to seconds.

https://github.com/hashicorp/consul/blob/0c3db76c922f1e5458ec3b5ece01509fb37f17ab/agent/structs/structs.go#L2220-L2236

There's some more info on this here:

https://github.com/hashicorp/consul/blob/0c3db76c922f1e5458ec3b5ece01509fb37f17ab/agent/structs/structs.go#L155-L161

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When using the API to create a new Lock Session with a `LockDelay` using a string of "5" as the `LockDelay` I get the following error:

`Request decode failed: time: missing unit in duration "5"`

Whereas a `5` as an integer is assumed to be seconds and `5000` is assumed nanoseconds.

Overall its a little confusing and I can't see any public documentation around this, so might be an idea to improve this or at least provide some API documentation detailing this.

cc @mkeeler