webui: Update job status when the job is cancelled; parallelize meteor method handlers; Change `SEARCH_SIGNAL` enums to strings instead of numbers.

[junhaoliao]

References

Internally, it was discovered that:

1. When the job is cancelled by the user, the progress bar is still running.
2. There are delays in UI updates after a search job is submitted.
3. It is hard to debug a job's lastSignal, which is a aggregated state from the last request by user and the last response by server , because the field appears as an base-10 integer in the MongoDB. Decimal to hex conversion is usually needed before developers can infer the status code.

Description

1. Update job status when the job is cancelled.
2. Parallelize meteor method handlers.
3. Change SEARCH_SIGNAL enums to strings instead of numbers.

Validation performed

1. cd <PROJECT_ROOT>; task
2. cd ./build/clp-package/sbin; ./start-clp.sh
3. ./compress.sh ~/samples/hive-24hr/i-00c90a0f/
4. Opened the WebUI address in a browser.
5. Started a query with string 123 and observed the job finished and the results are displayed in the results table.
6. Started another query with string 1234 and cancelled the job before the job finished. Observed the progress bar to disappear and an error message is displayed as "The search results are inconclusive because the user cancelled the job.".
7. Checked the results-metadata in MongoDB using MongoDB Compass and verified the job's last signal is a string of resp-done than an integer.