UCT/DC: Fix DCI pending allocation scheme - v1.3.x

[brminich]

Fix for a problem found with application.
The application crashed due to the following assertion:
dc_ep.c:136 Assertion `ep->dci == UCT_DC_EP_NO_DCI' failed

The problem happened during the following flow:

1. TX resources are exhausted
2. Some operation is added to the pending queue (waiting for DCI allocation)
3. Another operation has been invoked for the same ep (which forced ep to take DCI, but it was not descheduled from DCI allocation pending queue)
4. The assertion happened

Picked from master #2740

[swx-jenkins1]

Test PASSed.
See http://bgate.mellanox.com/jenkins/job/gh-ucx-pr/4943/ for details.

[mellanox-github]

Test FAILed.
See http://hpc-master.lab.mtl.com:8080/job/hpc-ucx-pr/7351/ for details (Mellanox internal link).

[brminich]

The issue looks unrelated - submitted #2746

bot:mlx:retest

[mellanox-github]

Test FAILed.
See http://hpc-master.lab.mtl.com:8080/job/hpc-ucx-pr/7353/ for details (Mellanox internal link).

[mellanox-github]

Test PASSed.
See http://hpc-master.lab.mtl.com:8080/job/hpc-ucx-pr/7354/ for details (Mellanox internal link).

[brminich]

@hoopoepg, plz review this one as well.