Refactor handler and tasks to use the same struct

[SoapHia]

In all the funcs inside tasks/tasks.go, the variables tsbridge.MetricConfig, tsbridge.Metrics, tsbridge.Config and storage.Manager are required repeatedly. Instead of passing them every time, a struct could be made to store these values. This would make the function calls much cleaner to read.

There is already a struct named web.Handler which stores these values, so the code should be refactored to use the same struct in both handlers.go and tasks.go. Currently, handlers.go is only used for AppEngine, whereas tasks.go functions are used in both AppEngine and non-AppEngine cases. This must be accounted for when refactoring.

See also: post by @nerdinary in #103 (comment)