Ch6. Sharing data between handlers could use context.Context

[dahernan]

Ch6, Page 159. The use of a map to store data could be replace it with context.Context

[matryer]

The real point here was to show off locking maps etc. I think I'll add a note in 2nd edition to talk about context.Context. I'll be sure to use the context package in the new chapters.

[matryer]

Thanks.