speed up statsd parser

[dgnorton]

Required for all PRs:

• CHANGELOG.md updated

Before

BenchmarkParse-4                                      50000         35152 ns/op
BenchmarkParseWithTemplate-4                          30000         45953 ns/op
BenchmarkParseWithTemplateAndFilter-4                 30000         50166 ns/op
BenchmarkParseWith2TemplatesAndFilter-4               20000         64277 ns/op
BenchmarkParseWith2Templates3TagsAndFilter-4          20000         81050 ns/op

After

BenchmarkParse-4                                  100000         20525 ns/op
BenchmarkParseWithTemplate-4                      100000         20438 ns/op
BenchmarkParseWithTemplateAndFilter-4             100000         20692 ns/op
BenchmarkParseWith2TemplatesAndFilter-4           100000         20724 ns/op
BenchmarkParseWith2Templates3TagsAndFilter-4      100000         22354 ns/op

[sparrc]

this part where the template gets applied should be outside of the p == nil if-statement

[sparrc]

LGTM