indexer: Remove worker-specific request queue

indexer: Remove worker-specific request queue

- Each request is done in a single connection and indexer master disconnects
from worker after indexing job is complete.
- No need to keep a worker queue since it always have length of 1. Instead,
keep a pointer to indexer_request for each worker connection.
- If indexing is in progress for a user, don't try to use the same connection
when a new request arrives for the same user. Instead, move the request to the
back of the queue and wait for the in progress request to finish then
create a new connection and submit request to worker.