Enumeration in worker executor service

[vigoo]

Create a new service trait and implementation in worker-executor-service-base with two public operations (in pseudo code):

• get-running-workers(template_id, filter) -> vec<metadata>
• scan-workers(template_id, filter, precise, cursor) -> option<(cursor, vec<metadata>)>

This service should have a default “live implementation”, which:

• uses the ActiveWorkers to enumerate all workers in memory
• uses a combination of Redis and the WorkerService to perform the SCAN to lazily enumerate all workers

Both operations should use a common implementation of applying the filter to the chunk of worker metadata.

We should have a precise bool flag (or some mode enum with two modes for now) with the following meaning:

• When it is true, scan-workers must check every worker metadata’s cached part (WorkerStatusRecord in the code) to see if it’s oplog_index is the same as the oplog’s size (requires a XLEN redis operation - to be done through OplogService). If not, an up-to-date WorkerStatusRecord needs to be generated calculate_last_known_status. This needs to be done before applying the filters (at least that part of the filter which is related to status)