add s3 circuit breaker support for 'simultaneous request count' and 'simultaneous request bytes' limitations

configure s3 circuit breaker by 'command_s3_circuitbreaker.go':
usage eg:
# Configure the number of simultaneous global (current s3api node) requests
s3.circuit.breaker -global -type count -actions Write -values 1000 -apply

# Configure the number of simultaneous requests for bucket x read and write
s3.circuit.breaker -buckets -type count -actions Read,Write -values 1000 -apply

# Configure the total bytes of simultaneous requests for bucket write
s3.circuit.breaker -buckets -type bytes -actions Write -values 100MiB -apply

# Disable circuit breaker config of bucket 'x'
s3.circuit.breaker -buckets x -enable false -apply

# Delete circuit breaker config of bucket 'x'
s3.circuit.breaker -buckets x -delete -apply

weed/config/s3_config.go

@@ -0,0 +1,16 @@
+package config
+
+import "strings"
+
+var (
+	CircuitBreakerConfigDir  = "/etc/s3"
+	CircuitBreakerConfigFile = "circuit_breaker.json"
+	AllowedActions           = []string{"Read", "Write", "List", "Tagging", "Admin"}
+	LimitTypeCount           = "count"
+	LimitTypeBytes           = "bytes"
+	Separator                = ":"
+)
+
+func Concat(elements ...string) string {
+	return strings.Join(elements, Separator)
+}