* Add --workers flag to mmctl import process to control concurrency
The bulk import worker count was hardcoded to runtime.NumCPU(), causing
high database load on the master during imports on live systems. This is
particularly impactful for incremental Slack imports where all users are
re-imported each time, generating 8-15 DB operations per user against
the master (due to LockToMaster).
The new --workers flag allows administrators to reduce concurrency
(e.g., --workers 1) to minimize impact on live users at the cost of
longer import duration. Defaults to 0 which preserves the existing
runtime.NumCPU() behavior.
* Add max workers limit, capped at CPU Count * 4