change: 优化, 使用"+"代替Sprintf

[n4mine]

Sprintf性能太差, RrdFileName、FormRrdCacheKey均为高频使用的函数。
附benchmark

=== RUN   Test_RrdFileName
--- PASS: Test_RrdFileName (0.00s)
=== RUN   Test_FormRrdCacheKey
--- PASS: Test_FormRrdCacheKey (0.00s)
goos: darwin
goarch: amd64
pkg: github.com/open-falcon/falcon-plus/modules/graph/g
Benchmark_RrdFileName-4            	10000000	       127 ns/op
Benchmark_RrdFileName_orig-4       	 3000000	       506 ns/op
Benchmark_FormRrdCacheKey-4        	20000000	        90.4 ns/op
Benchmark_FormRrdCacheKey_orig-4   	 5000000	       324 ns/op
PASS
ok  	github.com/open-falcon/falcon-plus/modules/graph/g	7.282s

[laiwei]

nice job 👍