fix(zipkin) don't assume ctx.KONG_* vars are set
fix(zipkin) don't assume ctx.KONG_* vars are set

Fixes #68

The plugin assumed that certain variables such as `ctx.KONG_ACCESS_START` were always set. There are occasions where this isn't true, for example an auth plugin might skip the access phase altogether.

The assumption provoked unwanted status reports. In #68 we can see an example in which the expected status code is 400 but because of this assumption we get 500 instead.

This wraps all accesses to ctx.KONG_* with conditionals, using other values such as ngx_now_mu() if they are not present.

There is some repetitiveness, but I resisted abstracting into a function to allow for short-circuiting the call to obtain the default value (i.e. to only call to ngx_now() when it was really needed).

Kong/kong,Kong/kong,Kong/kong