replace context.WithValue string key to typed struct{} key

[lysu]

Current TiDB many places uses a string as the context key which leads to allocations.

for example:

tidb/store/tikv/coprocessor.go

Line 52 in eae30eb

ctx = context.WithValue(ctx, txnStartKey, req.StartTs)

tidb/session/session.go

Line 505 in 7f3bff8

ctx = context.WithValue(ctx, execdetails.CommitDetailCtxKey, &commitDetail)

https://golang.org/pkg/context/#WithValue

To avoid allocating when assigning to an interface{}, context keys often have concrete type struct{}. Alternatively, exported context key variables' static type should be a pointer or interface.

https://www.dropbox.com/s/9u74uvpq8vowwdo/Screenshot%202019-07-30%2012.39.20.png?dl=0

instead, we'd better using type xKey struct{} just like opentracing does https://github.com/opentracing/opentracing-go/blob/master/gocontext.go#L12 to reduce alloc

[lauhg]

I can work on this if nobody else is.