Tighten HFileLink api to enable non-snapshot uses

In HBASE-12332 we'd like to use the FileLink's IO redirecting powers but want to be able to specify arbitrary alternate link paths and not be tied to the SnapshotFileLink file pattern (aka, table=region-hfile).

To do this we need change the constructors and some internals so that it is more generic. Along the way, we remove the FileStatus constructor arguments in favor of Path's and reduce the number of ways to create HFileLinks, and tighten up the scope privacy of many methods.