Fixes#7721
When S3 server is configured with a filerGroup, it creates a MasterClient
to enable dynamic filer discovery. However, the KeepConnectedToMaster()
background goroutine was never started, causing GetMaster() to block
indefinitely in WaitUntilConnected().
This resulted in the log message:
WaitUntilConnected still waiting for master connection (attempt N)...
being logged repeatedly every ~20 seconds.
The fix adds the missing 'go masterClient.KeepConnectedToMaster(ctx)'
call to properly establish the connection to master servers.
Also adds unit tests to verify WaitUntilConnected respects context
cancellation.
d80d8be012