Locking by label from 2 jobs exact the same time causes lock up of one of them

The Situation:
- We have 2 pipeline jobs starting the exact same time running on 2 different agents.
- We have 3 registered lockable resources with the same label.

This is what happens:
- Both jobs reach the lock command for a label exact the same time
- Both jobs get the same resource back and try to lock it
- Only one job continues with the locked resource, the other one waits.

What should have happened:
- Both jobs reach the lock command for a label exact the same time
- Each job gets another resource name back and locks it
- Both jobs continue each with their locked resource

Shared pipeline code for both jobs:

```
lock(label: 'STP-' + CFG_NR, variable: 'STPCLONE') {
echo "Current Lock on Clones: " + env.STPCLONE
.... Lot more code ....
}
```

Our Lockable Resource configuration:
(Manually entered in Jenkins configuration!)

```
Name: STP-S2D01
Labels: STP-001
```

```
Name: STP-S4D04
Labels: STP-001
```

```
Name: STP-S1D04
Labels: STP-001
```

Job 1 Console

```
[Pipeline] lock
22:43:19 Trying to acquire lock on [Label: STP-001, Quantity: 1]
22:43:19 Lock acquired on [Label: STP-001, Quantity: 1]
[Pipeline] {
[Pipeline] echo
22:43:19 Current Lock on Clone: STP-S2D01
```

Job 2 Console

```
[Pipeline] lock
22:43:19 Trying to acquire lock on [Label: STP-001, Quantity: 1]
22:43:20 [Label: STP-001, Quantity: 1] is locked, waiting...
06:40:13 Lock acquired on [Label: STP-001, Quantity: 1]
[Pipeline] {
[Pipeline] echo
06:40:13 Current Lock on Clone: STP-S2D01
```

Reproduce the issue with two pipeline jobs calling lock(label: 'STP-' + CFG_NR, variable: 'STPCLONE') against the three STP-001 resources, then compare both console logs. Investigate the label-based allocation and locking path responsible for returning STP-S2D01 to both jobs. Done means simultaneous jobs acquire different resources and both continue without waiting.

Written by the indexing model from the issue text.