Make Thanos Query wait for initial endpoint discovery before becoming ready

Problem:
We observed a race condition where Thanos Query components were marking themselves as ready before discovering any endpoints. This created a timing gap that could lead to query failures:

- Query pods become ready immediately upon startup
- Endpoint discovery happens asynchronously in the background
- Queries arriving between readiness and endpoint discovery fail

Solution:
This commit modifies the Thanos Query readiness behavior to wait for the initial endpoint discovery to complete before marking the pod as ready. This ensures that when a Query pod reports ready, it has already attempted to discover and connect to available endpoints.

Changes:
1. Added synchronization to EndpointSet:
   - Added firstUpdateOnce flag and firstUpdateChan channel to track first update completion
   - Added WaitForFirstUpdate() method to block until initial discovery completes

2. Modified Query startup sequence:
   - gRPC server now waits for WaitForFirstUpdate() before calling statusProber.Ready()
   - Leverages existing runutil.Repeat behavior which runs the update function immediately

3. Timeout protection:
   - Uses store response timeout or 30 seconds as default timeout
   - Logs warning if timeout occurs but still proceeds to ready state

4. Added comprehensive tests for the new WaitForFirstUpdate functionality

Impact:
- Positive: Eliminates the race condition where queries could be routed to Query pods that haven't discovered any endpoints yet
- Negative: Slightly increases startup time as pods won't be ready until endpoint discovery completes (typically <1s in normal conditions)

Signed-off-by: Pedro Tanaka <pedro.tanaka@shopify.com>

Author: pedro-stanaka