meta/redis: use smaller retry backoff in sentinel mode

[SandyXSD]

#1593

try = 0: sleep = 0
	if kill -9 (connection refused): write wait = 0
	if down host (i/o timeout): write wait = opt.WriteTimeout = 5s
min, max = opt.MinRetryBackoff, opt.MaxRetryBackoff = 20ms, 10s
try = 1; sleep ~= min << 1 / 2 + min = 40ms
try = 2; sleep ~= min << 2 / 2 + min = 60ms
...
try = 9; sleep ~= min << 9 / 2 + min = 5s
try = 10; sleep ~= min << 10 / 2 + min = 10s

Once = (10 + 5 + 2.5 + 1.25 + ...) + 11 * (write wait)
	= 20s or 75s
Total = Once * 3
	= 60s or 225s