UploadPack: refactor want validation

UploadPack: refactor want validation

Associate each RequestPolicy with an implementation of a
RequestValidator interface that contains the validation logic. The
checkWants method is only called if there are wants that were not
advertised, since clients may always request any advertised want
according to the git protocol. Calling the method only once at the
end of parsing the want list also means policy implementations can be
stateful, unlike the previous switch statement inside a loop.

For the special handling of unidirectional pipes, simply check
isBiDirectional() and delegate to other implementations if necessary.

Change-Id: I52a174999ac3a5aca46d3469cb0b81edd1710580