web/forms: change integer parsers to receive the base

web: fix redirect factories and improve 400/405/500 factories

web/forms: IsEmptyString() and IsNilOrEmptyString()

web: const.Allow assets.AssetHandler SetHeader

net/reconnect: fix handling of Temporary errors on IsFatal()

web/forms: introduce ErrMissing and add aliases to strconv errors

net/reconnect: Client: fix handling of context cancellation

net/reconnect: introduce WorkGroup interface

net/reconnect: make Client unbuffered

net/reconnect: fixes, improvements, clean up