[feat] Improve state selection criteria in the `--distribute` and the `--flex-alloc-nodes` option

[vkarak]

The key problem with those options currently is that there is no way to select a node that is exclusively in the idle state. As a result, nodes in idle+drain etc. states will be selected and jobs will be blocked forever. This PR fixes this by introducing exclusive state selection. More specifically, the following changes are made:

1. The STATE argument implies exclusive state selection.
2. The old behaviour is preserved with STATE*, meaning that a node to be selected must be at least in state STATE.
3. The new special argument avail is added which will selected all the nodes available for running a job. For Slurm, these are nodes in IDLE, ALLOCATED or COMPLETING states.

Closes #2258.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 79.41176% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 86.65%. Comparing base (7e6514a) to head (b5bb0ea).

Files	Patch %	Lines
reframe/core/schedulers/__init__.py	69.56%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #3140 +/- ## =========================================== - Coverage 86.66% 86.65% -0.02% =========================================== Files 61 61 Lines 12092 12105 +13 =========================================== + Hits 10480 10489 +9 - Misses 1612 1616 +4 ```

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