INFILE deregister functions

[julienschmidt]

Provide functions to remove ReaderHandler and filepaths from the
registry

[arnehormann]

LGTM, but I'd prefer not storing a map but having a callback function func(name string) io.Reader and a default implementation for reading a named file from the filesystem. That way users can handle this themselves and we can simplify the code and get rid of the dsn parameter. Anyone wanting to limit access to files can still do it by having an own whitelist and we only store and use one function (which should probably stored in the connection struct and not globally).