xcvrd timeout support for DellEMC S6000

[paavaanan]

- What I did
Added xcvrd support for DellEMC S6000 switches

- How I did it

• Added timeout support in get_transceiver_change(). Here we used time.time() library which is subject to variance as per system clock adjustments. This helps us to detect modified time variations.

• Handled timeout value and update port_dict within the given timeout value. Only incremental updates are sent to xcvrd.

• If no change happned within timeout value sleep with 1 second granularity..

-- How to verify it
Attached UT logs with instrumented code-change
unit-test-logs-nov.txt

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[jleveque]

This PR seems to be exactly the same as #2560. The only difference is that this one is against the 201811 branch, whereas the other is against master. If there are truly no differences between the PRs, please close this PR and add the "Request_for_201811_branch" label to the other PR. Once it is merged into master, the maintainer of the 201811 branch will cherry-pick the commit into the 201811 branch.

[Anandaraj-Maharajan]

@jleveque from Dell side we don't have permission to add the "Request_for_201811_branch" tag in the PRs. For 201811 branch we should reach out to Ying to add this tag? If not, can you tell who is the maintainer?

[jleveque]

@Anandaraj-Maharajan: Ying is the current maintainer of the 201811 branch. I will go ahead and add the label to the other PR and close this one.