Reduce high flush buffer contention

With many concurrent writers on a LSS, high flush buffer contention can lead to excessive calls to runtime.Gosched() in lsStore.ReserveSpaceMulti(...). This can lead to a lot of time spent in goroutine scheduling which degrades performance. This was observed in this recovery test.

For test TestShardRecoveryShared:

On MacBook (4 cores, 8 threads):

• GOMAXPROCS=4  -> ~16s
• GOMAXPROCS=8  -> ~20s
• GOMAXPROCS=16 -> ~250s

On GSI CI VM (4 cores, 16 threads):

• GOMAXPROCS=4  -> ~500s
• GOMAXPROCS=16 -> ~500s